A combined conscript and volunteer force , the National Army , was formed by the United States War Department in 1917 to fight in World War I . During World War II , the Army of the United States was formed as a successor to the National Army . The end of World War II set the stage for the ideological confrontation known as the Cold War . With the outbreak of the Korean War , concerns over the defense of Western Europe led to the establishment of NATO . During the Cold War , American troops and their allies fought communist forces in Korea and Vietnam ( see containment ) . The 1980s was mostly a decade of reorganization . The Army converted to an all-volunteer force with greater emphasis on training and technology . By 1989 , the Cold War was nearing its conclusion . The Army leadership reacted by starting to plan for a reduction in strength . After Desert Storm , the Army did not see major combat operations for the remainder of the 1990s . After the September 11 attacks , and as part of the War on Terror , U.S. and other NATO forces invaded Afghanistan in 2001 , replacing the Taliban government . The Army took part in the U.S. and allied 2003 invasion of Iraq
September 11 attacks
September 11 attacks
History of the United States Army