The opening of the West in the decades following the War of 1812 brought an important change in the organization of the Army. Experience having shown that infantry were at a distinct disadvantage when pitted against the fleetly mounted Indians, in 1832 a battalion of mounted rangers was organized to quell disturbances on the northwest frontier,[24] but this loosely knit force was replaced by a regiment of dragoons the following year.[25] The mounted arm had come to stay in the Army. [Footnote 24: Acts of April 5 and June 15, 1832 (_Military Laws_, pp. 322-323, 325-326).] [Footnote 25: Act of March 2, 1833 (_Military Laws_, pp. 329-330).] When the second Seminole War broke out in 1836, a second regiment of dragoons was organized.[26] And, as the war dragged through another inconclusive year, a reluctant Congress was forced to increase the size of existing line units and to authorize an additional regiment of infantry, the 8th. Meanwhile, increasing demands for surveying and mapping services resulted in the creation of the Corps of Topographical Engineers as a separate entity.[27] [Footnote 26: Act of May 23, 1836 (_Military Laws_, pp. 336-337).] [Footnote 27: Act of July 5, 1838 (_Military Laws_, pp. 341-349).] Meanwhile, the responsibilities of the Army in the opening of the West continued to increase, and in 1846 the Regiment of Mounted Riflemen was organized to consolidate the northern route to the Pacific by establishing and manning a series of posts along the Oregon Trail.[28] However, the outbreak of the War with Mexico postponed this mission. [Footnote 28: Act of May 19, 1846 (_Military Laws_, pp. 371-372).] At the start of the War with Mexico Congress leaned heavily on volunteer units, with the hard core of the Regulars remaining unchanged. But early in 1847 it was found necessary to add nine regiments of infantry and one regiment of dragoons.[29] Of the infantry unit's, eight were of the conventional type; the ninth was formed as the Regiment of Voltigeurs and Foot Riflemen. Theoretically, only half of this latter regiment was to be mounted. Each horseman was to be paired with a foot soldier who was to get up behind and ride double when speed was needed. In practice, however, none of the Voltigeurs were mounted; the entire unit fought as foot riflemen.[30] [Footnote 29: Act of February 11, 1847 (_Military Laws_, pp. 379-382).] [Footnote 30: MAHON, "History of the Organization of the United States Infantry," p. 16.] All of these new units proved merely creatures of the war, and the coming of peace saw a reduction to the old establishment of eight regiments of infantry, four of artillery, two of dragoons, and one regiment of mounted riflemen.[31] This organization remained substantially unchanged until 1855.[32] [Footnote 31: Official Army Register, 1848.] [Footnote 32: UPTON, p. 223.] Organization of the Militia