A survey of military histories published by the Department of the Army reveals the fact that there are many important incidents in early American military life not covered by official publications. Among them is the expedition of United States troops into Mexico in 1916, generally known as the Mexican Punitive Expedition, which was undertaken in an effort to put an end to the loss of American lives and property caused by Mexican bandit raids and depreciations across and along the border. This monograph covers briefly the period of the expedition and the benefits gained from that experience. The story is confined primarily to the military aspects of the campaign with only brief references to early American-Mexican relations and to border concentration of the Army, Militia, and National Guard, This was the first such mobilization of troops in the history of the United States Army and provided experience that proved of great value in the World War so soon to follow. Mobilization, the replacement system, and demobilization are treated at length in studies prepared in the Office of the Chief of Military History under the respective titles, History of Military Mobilization in the United States, History of the Personnel System, United States Army, and History of Demobilization, United States Army, Official records of the expedition now stored in the National Archives, Washington, D.C., were used in the preparation of this narrative, supplemented by research into the Wilson, Borah, and Daniels papers in the Library of Congress, and by interviews with participants of the expedition and students of that period of American history. Secondary sources were used to fill gaps when official records were not available and to highlight incidents not otherwise of record. A series of typical photographs has been inserted at pertinent places to illustrate the equipment, uniforms, and means of transportation of the several branches of service and locale over which the troops operated. F