WAR ATLAS MILITARY HISTORY MAGAZINE · WWI — WWII
Flanking manoeuvre
1. Lead
Attacking the side, not the front
A flanking manoeuvre is an attack directed against the side — the flank — of an enemy force, rather than against its front, where its defences are strongest. It is one of the oldest and most effective ideas in warfare, and it rests on a simple observation: no army can be strong everywhere.
An army that is facing forward, with its weapons, its fortifications and its attention directed at the enemy in front of it, is weak on its sides and weaker still at its rear, where its supply lines, its headquarters and its reserves lie.
A commander who can move around the enemy’s flank, unseen and unopposed, and strike it from the side can destroy an army that is far larger than his own, because he is attacking it where it cannot defend itself. The history of warfare is, in large part, a history of flanking manoeuvres: from Hannibal’s double envelopment at Cannae in 216 BC, through the German “sickle cut” of 1940, to the landing at Inchon in 1950, the great victories have, again and again, been won by going around the side.
Why the flank is the key to victory
The flanking manoeuvre matters because a frontal attack, however brave, is the most expensive way to win a battle. An army that attacks frontally runs into the enemy’s strongest defences, its minefields, its machine guns and its artillery, and it pays for every metre of ground with lives.
An army that flanks, by contrast, avoids the enemy’s strength and strikes at its weakness, and it can win with far fewer casualties. The flanking manoeuvre is also, above all, a matter of movement: it requires an army that can move faster than its enemy, that can march around the flank, through ground the enemy believes to be impassable, and that can arrive at the enemy’s side before the enemy can turn to meet it.
The commanders who mastered the flanking manoeuvre — Hannibal, Napoleon, Erwin Rommel, Douglas MacArthur — were all, above all, masters of movement, and their victories were victories of speed and of surprise.
2. Key Facts
| Definition | An attack against the side (flank) of an enemy force, rather than its front |
| Variants | Single envelopment (one flank); double envelopment (both flanks, as at Cannae); turning movement (going around the flank to threaten the rear); vertical envelopment (airborne landing behind the enemy) |
| Classic examples | Cannae, 216 BC (double envelopment); the German “sickle cut”, May 1940; El Alamein, November 1942; Inchon, September 1950 |
| Requirements | Speed, surprise, accurate intelligence on the enemy’s flank, and a force that can move faster than the enemy can react |
| Counter | Refusing the flank (bending the line back), keeping a mobile reserve, and guarding the supply lines |
3. Background
The geometry of battle
Every army in battle has a front, two flanks and a rear, and the geometry of the battlefield is the geometry of those four sides. The front is where the army’s strength is concentrated: its infantry, its artillery, its fortifications and its attention are all directed forward, at the enemy it can see.
The flanks are where the army is weakest: they are the ends of the line, where the troops are thinner, where the artillery cannot fire, and where the army’s supply lines run back to its bases. The rear is where the army is most vulnerable: it holds the headquarters, the supply depots, the hospitals and the reserves, and if the rear is attacked, the army cannot fight, because it has no supplies, no orders and no way out.
The flanking manoeuvre is the art of exploiting this geometry: of avoiding the front, where the enemy is strong, and of striking the flank or the rear, where the enemy is weak. The commander who understands the geometry of battle understands that the way to win is not to push harder against the enemy’s strength, but to go around it.
From the phalanx to the tank
The flanking manoeuvre is as old as organised battle, and it has changed with every change in weapons and in movement. In the ancient world, the phalanx — the dense formation of spearmen that was the main weapon of the Greek and Macedonian armies — was strong in front and weak on its flanks, and the great captains of antiquity, from Epaminondas at Leuctra in 371 BC to Hannibal at Cannae, won their victories by striking the flank of the phalanx.
In the age of Napoleon, the flanking manoeuvre was made by marching columns, which could move faster than the enemy and arrive on its flank before it could react. In the age of the tank, the flanking manoeuvre was made by armoured divisions, which could move at 40 kilometres an hour and could go around the enemy’s flank in a day.
And in the age of the aircraft, the flanking manoeuvre could be made vertically, by paratroops and by helicopter, landing behind the enemy’s lines. The weapons have changed, but the geometry has not: the flank is still the weak point, and the commander who can reach it still wins.
4. Opposing Forces
The flanker and the flanked
A flanking manoeuvre is a contest between two kinds of force: the flanking force, which must be fast, mobile and self-sufficient, and the flanked force, which must be strong enough in front to hold the enemy’s attention, and weak enough on its flank to be surprised.
The flanking force is usually the smaller of the two, because it must move quickly and because it cannot carry heavy artillery or large supply trains with it. Its strength is its speed and its surprise, and its weakness is that, if it is detected, it can be cut off and destroyed, because it is far from its own bases and because it has exposed its own flank in the act of flanking.
The flanked force is usually the larger, but it is larger in the wrong place: its strength is in front, and its flank is thin. The table below sets out the two sides at Cannae, the classic example of the double envelopment.
Cannae, 216 BC: the two sides
| Carthage (Hannibal) / Carthage (Hannibal) | Rome | |
|---|---|---|
| Strength | About 50,000 men (about 40,000 infantry, 10,000 cavalry) | About 80,000 men (about 70,000 infantry, 10,000 cavalry) |
| Plan | Double envelopment: the centre gives way, the wings close in, the cavalry returns to attack the rear | Frontal attack: break the Carthaginian centre by weight of numbers |
| Cavalry | Superior: the Numidian and Spanish cavalry drive off the Roman cavalry, then return to attack the Roman rear | Inferior: the Roman cavalry is driven from the field and cannot return |
| Result | About 50,000–70,000 Romans killed, including 80 senators; Hannibal loses about 6,000 men | The worst defeat in Roman history; Rome survives, but only by refusing to surrender |
Cannae is the classic example because it shows, in its purest form, what a flanking manoeuvre can do. Hannibal, with fewer men, destroyed an army that was half as large again as his own, by attacking it from the sides and from the rear, and by turning its own strength — its weight of numbers, pressing forward into the centre — into a trap.
The Roman army, packed into a dense mass, could not turn, could not fight back, and could not escape, and it was slaughtered where it stood. The lesson of Cannae has been studied by every general since, and it is still taught in staff colleges today: never let your flanks be turned, and never advance so far that you cannot turn to meet an attack from the side.
5. The Battle
Cannae, 2 August 216 BC: the double envelopment / Cannae, 2/8/216 TCN: vòng vây kép
The Battle of Cannae was fought on 2 August 216 BC, in southern Italy, between the Carthaginian army of Hannibal and the Roman army of the consuls Lucius Aemilius Paullus and Gaius Terentius Varro. The Romans, with about 80,000 men, outnumbered Hannibal’s 50,000, and they planned to win by a frontal attack: their infantry, in a dense mass, would drive straight through the Carthaginian centre. Hannibal’s plan was the opposite.
He placed his weakest troops — the Gauls and the Spaniards — in the centre, in a crescent that bulged towards the Romans, and he placed his best troops — the Libyan infantry — on the wings, in columns, and his cavalry on the far wings.
When the Romans attacked, the Carthaginian centre gave way, slowly, drawing the Romans forward into the crescent, and the Libyans on the wings turned inwards, attacking the Romans from the sides. At the same time, the Carthaginian cavalry, having driven the Roman cavalry from the field, returned and attacked the Romans from the rear. The Roman army was surrounded on all four sides, and it was destroyed.
About 50,000 to 70,000 Romans were killed, including both consuls — Paullus was killed, and Varro escaped with a handful of men — and about 10,000 more were taken prisoner. Hannibal lost about 6,000 men. Cannae remains, more than 2,000 years later, the most complete victory ever won by a flanking manoeuvre, and it is the model against which every double envelopment since has been measured.
The trap in the centre
The key to Cannae was the centre. Hannibal’s centre was deliberately weak, and it was designed to give way, to draw the Romans forward, and to turn their own advance into a trap. The Romans, who believed that they were winning, because the Carthaginian centre was retreating, pressed forward, and in doing so they packed themselves into a dense mass, in which they could not use their weapons, could not turn, and could not escape.
The centre was not a defeat: it was a bait, and the Romans took it. The lesson is that a flanking manoeuvre does not always begin on the flank: it can begin in the centre, with a feigned retreat, and the flank attack is the blow that completes the trap.
The sickle cut, May 1940: the flank that decided a war
The most famous flanking manoeuvre of the Second World War was the German “sickle cut” of May 1940, the plan that defeated France in six weeks. The French and British armies expected the German attack to come through Belgium, as it had in 1914, and they advanced into Belgium to meet it, leaving their flank — the Ardennes forest, which they believed to be impassable to tanks — weakly held.
The German plan, devised by General Erich von Manstein and adopted by Hitler, was to attack through Belgium with one army group, as a feint, and to send the main force — seven panzer divisions — through the Ardennes, around the flank of the Allied armies, and then to the Channel coast, cutting the Allied armies off from their bases. The plan worked.
The panzers crossed the Ardennes in three days, crossed the Meuse at Sedan on 13 May, and reached the Channel at Abbeville on 20 May, cutting off the British and French armies in Belgium. The Allied armies, trapped against the sea, were evacuated at Dunkirk, and France surrendered on 22 June.
The sickle cut was a flanking manoeuvre on the scale of an entire campaign, and it decided the war in the west in six weeks. It succeeded because the French believed the Ardennes to be impassable, and because the Germans, who had practised the crossing, knew that it was not.
Inchon, September 1950: the flank from the sea
The most famous flanking manoeuvre of the post-war period was the landing at Inchon, in Korea, on 15 September 1950. The North Korean army had driven the United States and South Korean forces back to the Pusan Perimeter, in the south-east corner of Korea, and it was on the verge of victory.
General Douglas MacArthur, the commander of the United Nations forces, proposed a landing at Inchon, on the west coast of Korea, far behind the North Korean lines, to cut the North Korean supply lines and to retake Seoul. The plan was opposed by almost every other senior officer, because Inchon had one of the largest tidal ranges in the world — the tide rose and fell by about 10 metres — and because the landing could only be made on a few days a month, at high tide, and because the North Koreans, if they had defended the port, could have destroyed the landing force.
MacArthur insisted, and the landing was made on 15 September, with 75,000 men, and it succeeded. The North Korean army, cut off from its supplies, collapsed, and the United Nations forces retook Seoul within two weeks. Inchon was a flanking manoeuvre from the sea, and it is the classic example of the turning movement: an attack that does not meet the enemy’s front at all, but goes around it, by sea, and strikes at its rear.
6. Turning Point
The moment the flank is turned
The turning point in any flanking manoeuvre is the moment at which the flank is turned — the moment at which the flanking force arrives on the enemy’s side, and the enemy, which has been facing forward, realises that it is being attacked from a direction it did not expect.
At Cannae, that moment came when the Libyan infantry on the wings turned inwards, and the Roman army, which had been pressing forward, found itself attacked from the sides. In the sickle cut, that moment came on 20 May 1940, when the German panzers reached the Channel at Abbeville, and the Allied armies in Belgium realised that they were cut off.
At Inchon, that moment came on 15 September 1950, when the landing force came ashore and the North Korean army realised that its supply lines had been cut. In each case, the moment was a psychological as well as a physical one: the enemy’s morale broke, not because it had been defeated in battle, but because it had been outmanoeuvred, and because it no longer knew which way to face.
Speed and surprise
The turning point in a flanking manoeuvre depends, above all, on speed and on surprise, and the history of the tactic is the history of commanders who had them and commanders who did not. Hannibal had both at Cannae: his cavalry was faster than the Roman cavalry, and his plan was a surprise, because the Romans did not expect the Carthaginian centre to give way.
The Germans had both in the sickle cut: their panzers were faster than the French and British armies, and their attack through the Ardennes was a surprise, because the French believed the forest to be impassable. MacArthur had both at Inchon: his landing was a surprise, because the North Koreans did not expect a landing at a port with such tides, and it was fast, because the landing force moved inland before the North Koreans could react.
The flanking manoeuvre that is slow, or that is expected, is not a flanking manoeuvre at all: it is a frontal attack, made from the side, and it will fail.
7. Casualties and Losses
The cost of being flanked
The casualties of a flanking manoeuvre fall, above all, on the army that is flanked, and they are often far heavier than the casualties of a frontal battle, because an army that is attacked from the flank cannot fight back, cannot retreat, and cannot surrender in an orderly way.
At Cannae, the Romans lost about 50,000 to 70,000 men killed, out of an army of about 80,000 — a death rate of more than 60 per cent — while Hannibal lost about 6,000 men, a death rate of about 12 per cent.
The figures are estimates, based on the accounts of the ancient historians Polybius and Livy, who disagree about the exact numbers, but the order of magnitude is not in dispute: the flanked army was destroyed, and the flanking army survived.
In the sickle cut of 1940, the French and British armies lost, in the campaign as a whole, about 360,000 men killed, wounded and missing, and about 1.9 million more were taken prisoner, while the German losses were about 150,000 in total.
The figures are approximate, and they cover the whole campaign, not only the flanking manoeuvre, but they show the same pattern: the army that is flanked loses far more than the army that flanks.
The cost of flanking
The flanking force also pays a price, and it is a price that is often forgotten, because the flanking force is the victor. The flanking force must march further, and faster, than the enemy, and it must do so with fewer supplies, because it cannot carry a large supply train with it.
Its men are tired when they arrive, and if the flanking manoeuvre fails — if the enemy turns in time, or if the flanking force is detected and cut off — the flanking force is itself in danger of being destroyed. At Cannae, Hannibal’s cavalry had to ride around the Roman army, and it arrived at the Roman rear exhausted.
In the sickle cut, the German panzer divisions advanced so fast that they outran their supply lines, and they were, for a time, vulnerable to a counterattack that never came. The flanking manoeuvre is a gamble, and the stakes are the flanking force itself.
8. Map and Geography
The ground that makes flanking possible
The flanking manoeuvre depends, above all, on the ground, because it is the ground that determines whether a flank can be turned. The ground that favours flanking is open ground, through which an army can move quickly, and ground that the enemy believes to be impassable, through which an army can move unseen.
The Ardennes, in 1940, was ground of the second kind: the French believed it to be impassable to tanks, and the Germans, who had practised the crossing, knew that it was not. The desert, in North Africa, was ground of the first kind: it was open, and the armies could move around each other’s flanks at will, which is why the desert war was a war of flanking manoeuvres, and why Erwin Rommel and the British commanders fought a war of movement in which the front line changed every week.
The sea, at Inchon, was ground of a third kind: it was a way around the enemy’s flank that the enemy could not guard, because it could not be everywhere at once. The commander who understands the ground understands where the flank can be turned, and the commander who does not understands it too late.
The flank and the map
The map is the flanking commander’s most important tool, and the history of the flanking manoeuvre is, in large part, a history of maps. Hannibal’s map of Italy, in 216 BC, was a map drawn from memory and from the reports of guides, and it was good enough to bring him to Cannae.
Manstein’s map of France, in 1940, was a map of roads and of forests, and it showed him that the Ardennes could be crossed. MacArthur’s map of Korea, in 1950, was a map of tides and of harbours, and it showed him that Inchon could be landed at.
In each case, the map showed the commander something that the enemy did not see: a way around the flank, through ground that the enemy believed to be closed. The flanking manoeuvre is, in this sense, an act of imagination as much as an act of movement: it is the ability to see, on the map, a road that the enemy does not see, and to take it.
9. Outcome and Legacy of Flanking manoeuvre
The tactic that never grows old
The flanking manoeuvre has never grown old, and it has never been superseded, because the geometry of battle has never changed. The weapons have changed — from the spear to the tank to the helicopter — but the flank is still the weak point, and the commander who can reach it still wins.
The flanking manoeuvre decided the Second World War in the west, in six weeks in 1940, and it decided the Korean War, in two weeks in 1950. It decided the Six-Day War of 1967, in which the Israeli army, by a series of flanking manoeuvres, defeated three Arab armies in six days.
And it decided the Gulf War of 1991, in which the Coalition armies, by a great flanking movement through the desert — the “left hook” — destroyed the Iraqi army in 100 hours. The flanking manoeuvre is the oldest tactic in warfare, and it is still, today, the tactic that wins wars.
The defence against flanking
The legacy of the flanking manoeuvre is also a legacy of defence, because every army since Cannae has tried to protect its flanks. The methods are well known: refuse the flank, by bending the line back; keep a mobile reserve, which can move to meet a flanking attack; guard the supply lines, by placing troops along them; and, above all, maintain reconnaissance, so that a flanking force is detected before it arrives.
The armies that have done these things have survived, and the armies that have not have been destroyed. The Roman army, after Cannae, learned to protect its flanks, and it never again suffered a defeat like Cannae. The French army, after 1940, learned to protect its flanks, and it never again left a forest unguarded.
The flanking manoeuvre is, in the end, a test of the defender as much as of the attacker: it tests whether the defender has thought about its flanks, and whether it has prepared for the attack that comes, not from the front, but from the side.
10. Commanders
The masters of the flank
Hannibal Barca (Carthage). The Carthaginian general who won the Battle of Cannae in 216 BC, the most complete victory ever won by a flanking manoeuvre. Hannibal, who had crossed the Alps with elephants to invade Italy, understood the flank better than any general before him, and his double envelopment at Cannae remains the model for every flanking manoeuvre since. He won every battle he fought in Italy, but he lost the war, because Rome, which had learned from Cannae, refused to meet him in battle again, and attacked Carthage itself instead.
Erich von Manstein (Germany). The German general who devised the “sickle cut” plan of May 1940, the flanking manoeuvre that defeated France in six weeks. Manstein, who was, by common consent, the ablest German general of the Second World War, understood that the way to defeat France was not to attack its strongest defences, on the Maginot Line, but to go around them, through the Ardennes. He went on to command on the Eastern Front, where his counter-offensives and his flanking manoeuvres were among the finest of the war.
The men who used the sea and the desert
Douglas MacArthur (United States). The American general who commanded the landing at Inchon in September 1950, the most famous flanking manoeuvre from the sea in modern history. MacArthur, who had commanded in the Pacific in the Second World War, understood that the way to defeat the North Korean army was not to attack it frontally, at Pusan, but to go around it, by sea, and to cut its supply lines. The landing at Inchon was opposed by almost every other senior officer, and it succeeded because MacArthur insisted on it.
Field Marshal Erwin Rommel (Germany). The German general who was the master of the flanking manoeuvre in the desert war in North Africa in 1941 and 1942. Rommel’s victories at Gazala and at Tobruk were won by flanking manoeuvres through the open desert, and his defeat at El Alamein was, in part, a defeat by a flanking manoeuvre, as the British Eighth Army turned his flank and drove him back. Rommel’s career in the desert is the classic example of a war of movement in which the flank was everything.
Frequently Asked Questions
What is a flanking manoeuvre?
An attack against the side (flank) of an enemy force, rather than its front, where its defences are strongest.
What is a double envelopment?
A flanking manoeuvre in which both flanks of the enemy are attacked at once, surrounding it on all sides. The classic example is Cannae, 216 BC.
What was the “sickle cut”?
The German plan of May 1940, in which the main force attacked through the Ardennes, around the flank of the French and British armies, and reached the Channel coast, cutting the Allied armies off from their bases. France surrendered six weeks later.
What was the landing at Inchon?
An amphibious landing at Inchon, on the west coast of Korea, on 15 September 1950, far behind the North Korean lines, which cut the North Korean supply lines and led to the recapture of Seoul within two weeks.
How do you defend against a flanking manoeuvre?
By refusing the flank (bending the line back), keeping a mobile reserve, guarding the supply lines, and maintaining reconnaissance so that a flanking force is detected before it arrives.
Is the flanking manoeuvre still used today?
Yes. It was used in the Six-Day War of 1967 and in the Gulf War of 1991, in the “left hook” through the desert, and it remains a central idea in modern military doctrine.
Related articles
Sources
- Wikipedia, “Flanking maneuver” — the definition, the variants, and the classic examples.
- Polybius, The Histories, Book III — the ancient account of the Battle of Cannae.
- Livy, History of Rome, Book XXII — the Roman account of Cannae.
- Wikipedia, “Battle of Cannae” — the numbers, the plan, and the casualties.
- Karl-Heinz Frieser, The Blitzkrieg Legend: The 1940 Campaign in the West (Naval Institute Press, 2005) — the sickle cut and the crossing of the Ardennes.
- Wikipedia, “Battle of Inchon” — the landing of 15 September 1950 and its consequences.
Further reading
- Adrian Goldsworthy, Cannae (Cassell, 2001) — the battle and its place in military history.
- Matthew B. Ridgway, The Korean War (Doubleday, 1967) — Inchon and the war in Korea.