 It was the tank that finally broke the trench lines to end both the war and the role of the horse soldier.The 15th served occupation duty after the war until June 1919 when it returned to the United States.The 5th Cavalry Regiment was deactivated on 18 October 1921, after 20 years of continuous active service, as part of the massive cutbacks in the Regular Army following "the war to end all wars".