But the general of to-day no less than the general of yesterday, would always know where his enemy is most vulnerable, and strike him at that point. In the spring of 1915, the line of least resistance for the German army was obviously to the east where the loose organization of the Russian army, lacking munitions, was stretched over a front of over a thousand miles. The French were better off in munitions, and their army and the British had a front of four hundred and fifty miles of intact trench line. It is estimated that in order to hold a battle front with modern troops, about three thousand men to the mile are required. This does not mean that there are three thousand actually on every mile; but counting the thin line in the trenches, the thicker line in the reserve trenches and the soldiers who are out of the trenches resting and the battalions in reserve and the reserve supplies of men in the depots who can promptly be brought into action. For example, to hold a mile of the famous Ypres salient might require double the number of men necessary to hold a mile where the lay of the ground was in the favor of your troops. Owing to the use of motor trucks and to railway trains, whenever there is an attack, concentration of men at any point is very rapid. Holding to this rule, the Germans maintained all through the summer of 1915, 1,500,000 men on their western front, and they had that number at least to spare for their eastern front. Field Marshal von Hindenburg said that by hammering he would get Warsaw, and he was to keep his word with stolid German persistence. Napoleon, who had depended upon the number of his guns, would have fully appreciated the Austro-German plan of action against Russia. The Russian army has been compared to cotton wool. The farther you went into Russia, the more cotton wool there was. The Russian army would yield, but there never seemed any end of it. Gaining a passive victory over the Russian army has also been compared to brushing the snow off the front doorstep. The more you brushed, the more snow banked up. Russia could afford to lose territory equivalent to the area of all France without having received a vital blow. Russia has plenty of room in which to retreat, as Napoleon learned. She is confident in the safety of her distances. When the enemy falls back she follows on his heels.