American Military History, Volume 1

The victory over Mexico, as well as the settlement of the Oregon boundary frontier in June 1846, added to the United States a vast territory that was to occupy the Army almost exclusively in the postwar years. For generations, the Army was to be the only force for law and order throughout thousands of square miles. First, the Army needed to explore this vast new conquest. In this task the Corps of Topographical Engineers played the leading role. Some knowledge of the new region had been gained by expeditions such as those of Capt. Benjamin L. E. Bonneville, Kearny, and 2d Lt. John C. Fremont; more had been gained during the Mexican War by "topogs" attached to Kearny's march to California and Wool's to Saltillo and after the war by Maj. William H. Emory's work with the Mexican Boundary Commission. But much still remained to be done.

The most significant and far-reaching explorations were those to locate routes for transcontinental railroads. The first effort was directed toward the southwest, seeking an "ice-free, mountain-free" route. In that area, the necessity for defense against Comanches, Apaches, and Navahos meant that most of the Army had to be stationed between San Antonio and Fort Yuma. Forts had to be constructed, roads built, rivers sought as avenues of supply, and Indian trails mapped. In 1853 Congress authorized similar explorations on a northern route to the Pacific from Chicago and a central route from St. Louis.