The problem presented involved not merely the selection of forces to be trained into armies but officers to do the training. By the provisions of the national defense act of June 3, 1916, Officers' Reserve Corps had been authorized. Rules and regulations for their organization were promulgated in July, 1916, and amended in March, 1917. Immediately upon the passage of the act, the building up of lists of reserve officers in the various sections of the Military Establishment was undertaken, with the result that at the end of the fiscal year some of the branches of the service had substantial lists of men available for duty in the event of call. The largest number of commissions were issued in the technical services, for which professional nonmilitary training was the principal requisite. The largest reserve corps was that in the Medical Department, in which more than 12,000 physicians were commissioned. The expansion of these technical services proceeded easily upon the basis of the reserve corps beginning, but the number of applicants for commissions in the strictly military or combatant branches of the service was relatively small. They consisted of men who had had military experience either in the Regular Army or the National Guard, and men who were graduates of schools and colleges affording military training, and of the training camps which for several years had been maintained at Plattsburg and throughout the country. Their number, however, was wholly inadequate, and their experience, while it had afforded the elements of military discipline, had not been such as was plainly required to train men for participation in the European war with its changed methods and conditions. The virtue of the law authorizing the Officers' Reserve Corps, however, became instantly apparent upon the declaration of war, as it enabled the department to establish officers' training camps for the rapid production of officers. [Sidenote: A series of officers training camps.] [Sidenote: Officers commissioned.]