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. [1] The site of this battle was probably Gaugamela, about 60 miles from the present Arbil, which is 40 miles from Mosul, on the Baghdad road. {33} HOW BATTLES ARE INFLUENCED Once troops are launched in battle their success or failure depends upon such influences as the commander can bring to bear, upon the co-operation of his subordinate commanders, and upon the moral and training of the troops engaged.