THE DECISIVE BLOW.--The preparatory action and the development usually take the form of a converging movement of separated forces, so timed as to strike the adversary's front and flank simultaneously, in order to threaten the enemy's line of communications, for the line of supply is as vital to the existence of an army as the heart to the life of a human being. "Perhaps no situation is more pitiable than that of a commander who has permitted an enemy to sever his communications. He sees the end of his resources at hand, but not the means to replenish them" (General Sir E. B. Hamley). The decisive blow will be delivered by the General Reserve, which will be secretly concentrated and launched as secretly as possible; and the commander of the whole force will so distribute his troops that about half his available force can be kept in hand for this decisive blow, on a part of the enemy's front if sufficient penetration has been effected, or on a flank. The point chosen becomes the vital { 32} point, and success there means success at all points. Once routed, the enemy must be relentlessly pursued and prevented from regaining order and moral.
A battle was fought in the year B.C. 331, nearly 2,300 years ago, at Arbela,[1] in Mesopotamia, the Eastern theatre of operations in the Great War of 1914-18, and it deserves study to show the eternal nature of the main principles which underlie the Art of War. Alexander the Great invaded the territories of Darius, King of the Medes and Persians, with the strategic aim of defeating his adversary's main armies in a decisive battle. The Macedonian forces were preceded by an Advanced Guard of Cavalry, and from information obtained by the Vanguard, Alexander was made aware of the strength and position of the Persian forces. By a careful reconnaissance of the ground in company with his Corps Commanders, Alexander was able to forestall a projected movement, and by advancing in two lines of battle in such a way that his troops could at any moment be thrown into a compact figure fringed with spears, which formed an impenetrable hedge against cavalry, he found a remedy for the disadvantages of the ground, which afforded no protection to either of his flanks. After advancing in these two lines Alexander manoeuvred his troops into a phalanx, or wedge-shaped figure, and this wedge he drove into the masses of the enemy to force the wings asunder. In spite of local reverses in parts of the field, the depth and weight of the main attack carried it through the enemy's forces: the survivors were captured or dispersed, and the victory was complete.
