Although the U.S. Army traces its roots to the Continental Army of the American Revolution, the United States did not have a consistent standing army until the nineteenth century. The federal government established a small standing army to protect the nation's frontiers. Although Congress occasionally expanded it during wartime, especially during the War of 1812 and the Mexican War, the U.S. Army remained relatively small until the Civil War, during which it became the largest in the world. Throughout this period, the Army was under the jurisdiction of the War Department.
Don Higginbotham, "Army, U.S.: Colonial and Revolutionary Eras," The Oxford Companion to American Military History ed. by John Whiteclay Chambers II (New York: Oxford University Press, 1999), 48-49; William B. Skelton, "Army, U.S.: 1783-1865," The Oxford Companion to American Military History, 49-51; Allan R. Millett and Peter Maslowski, For the Common Defense: A Military History of the United States of America (New York: MacMillan, 1984), 47-232.