Closing the gate - or - "the Classic pincer movement"
Closing the gate is a battlefield tactic which demonstrates a number of Principles of War. The most famous implementation of this strategy was by Hannibal at The Battle of Cannae. Hannibal ranks as one of the most able generals of all time, and using this tactic he managed to defeat a Roman force which massively outnumbered his own.
Hannibal faced the largest army ever fielded by Rome: 4 legions, 80,000 men. Hannibal had only half these troops. The two forces lined up in the conventional style of the time, with the infantry at the center and the cavalry on the flanks. Hannibal, however, held his most skilled and heavily armed infantry behind his front line of light cavalry. The Roman legionnaires, thinking they could easily overcome the Carthiginian front line by sheer force of numbers, threw their entire forces into driving the light infantry backwards. The very fact Hannibal chose to charge his troops was a surprise to the Romans, but what they did not know was that Hannibal had ordered his troops to fall back into a crescent shape to draw the Romans into a semi-circle. Meanwhile, the highly-skilled and numerically superior Carthiginian cavalry met with the Roman cavalry and drove them from the field.
Currently, the battlefield looks like this. Roman forces are in bold, Carthiginian in normal type. (C/C = Cavalry ; L = Legionnaire ; L = Light infantry ; H = Heavy infantry)
Little does the Roman commander know of the heavy infantry which the light is retreating towards. And although they can see they're gradually being drawn into a crescent because the Carthiginian line is stretched thinner than their own, they are sure they will prevail by force of numbers. This is point at which Hannibal chose to close the gate.
The Roman cavalry now driven from the field and the main Roman force drawn back to where the heavy infantry lie in waiting, the Carthiginian troops can move into the sides of their formation. The cavalry began to make repeated charges against the back of the Roman force - the gate was closed. Polybius said some 50,000-60,000 Romans perished inside that fortress of Carthiginian steel, more than any army has ever lost in a closing with the enemy in the history of the World.
What Principles of War have been employed in this age-old, pincer-movement strategy? What makes this strategy so effective is the Principle of Mass. Many misunderstand this principle, thinking it means one must deploy the maximum quantity of firepower and forces on the battlefield to be victorious. What really matters is the concentration of power that can be focused on a single point - by flanking the enemy and making them fight on multiple fronts, mass (power) has a geometric effect. Surprise is the ultimate tool in war - the real skill lies not only in doing that which the enemy does not expect, but appearing to do what he does expect. As the Carthiginian front line retreated as the Romans expected it to, they attacked with renewed vigour and increased carelessness. Thinking they had won, they were unprepared for the crushing blow delivered to them. The Roman commander would pay for his carelessness with his own life.
The pincer movement is one of the most basic tactics of infantry combat. Sun Tzu hinted that it was perhaps best not to close the gate as it would lead the enemy to fight with most ferocity, instead they should be allowed a path to flee by. In the Great Patriotic War, the Wehrmacht closed the gate repeatedly on millions of Red Army soldiers, leading to their surrender. In the Gulf War, the Americans did a similar thing (albeit on a smaller scale), to the Iraqi forces. They surrended, but had they not have done then the Allied forces would quickly have rained down ordinance from the air upon them. Such is the real danger of a closed gate in modern warfare. Closing it at speed is now much easier with the advent of mechanized warfare - tanks and armoured cars can travel at great speed along the enemy flanks. Making sure his forces are not outflanked is one of the primary concerns of any battlefield commander - allowing such to happen places him in a situation from which few have ever recovered.