The constantly increasing tendency toward employing mechanical and chemical means of warfare produced another difficulty. Before the United States entered the war, a total of 50 machine-guns was the standard equipment of an infantry division. But when the Armistice was signed the tables of organization gave each division 768 automatic rifles and 262 machine-guns, an increase in this type of equipment of more than 2,000 per cent. Furthermore, the General Staff of the A. E. F. was working on plans for the reorganization of infantry units which would have increased the number of automatic rifles in each company to 24—approximately one for every ten men—and which would have established a new equipment of 192 automatic rifles for each artillery brigade. It is scarcely necessary to point out that every additional automatic arm, with its insatiable appetite for cartridges, necessitated a corresponding increase in the requirements for ammunition and for ammunition supply. Before the United States entered the war, practically all field-artillery, including guns, howitzers, limbers, caissons, repair-wagons, and the like, was drawn by horses or mules, the Ordnance Department furnishing the harness and other horse equipment. The difficulty in obtaining an adequate supply of animals, however, together with the high rate of animal mortality, the constantly increasing weight of the guns, and the nature of the terrain, made necessary the wholesale motorization of the artillery, which was proceeding at an amazing rate when hostilities ended. Guns are now hauled by tractors; caissons and limbers have been displaced by motor ammunition-trucks; complete machine-shops, mounted on motor-trucks, supplant the old forge-limbers and battery and store wagons; machine-guns, instead of being packed on mules or drawn by horses, are usually moved to the front by various forms of motor transport and are often taken into action in tanks. Even the large-calibre field-pieces are now mounted on caterpillar tractors, which not only provide means of transportation for the guns but also the means for aiming them. These changes naturally brought others in their wake. The higher speed of motor-drawn artillery demanded rubber-tired wheels. The substitution of the automatic rifle, with its terrific burst of fire, for the ordinary shoulder rifle, entailed a tremendous increase in the capacity of the ammunition-trains. So, as the tools of war became more mechanically efficient, they became correspondingly more complicated to manufacture.