Lemnitzer graduated from the Coast Artillery School in 1921, and then served at Fort Adams in Rhode Island and in the Philippines. He was an instructor at West Point from 1926 to 1930. Lemnitzer served again in the Philippines from 1934 to 1935, and graduated from the United States Army Command and General Staff College in 1936. He was an instructor at the Coast Artillery School, and graduated from the United States Army War College in 1940. At the start of World War II Lemnitzer served with the 70th Coast Artillery Regiment and then the 38th Coast Artillery Brigade. In May 1941, Lemnitzer, then a colonel, was assigned to the War Plans Division of the Army staff, and then to the staff of the Army Ground Forces.
World War II
Lemnitzer was promoted to brigadier general in June 1942 and commanded the 34th Coast Artillery Brigade. He was subsequently assigned to General Dwight D. Eisenhower's staff, where he helped plan the invasions of North Africa and Sicily and was promoted to major general in November 1944. Lemnitzer was one of the senior officers sent to negotiate the Italian fascist surrender during the secret Operation Sunrise and the German surrender in 1945.
Lemnitzer was promoted to the rank of general and named commander of US Army forces in the Far East and of the Eighth Army in March 1955. He was named Chief of Staff of the United States Army in July 1957 and appointed Chairman of the Joint Chiefs of Staff in September 1960. As Chairman, Lemnitzer was involved in the Bay of Pigs crisis and the early years of United States involvement in the Vietnam War. He was also required to testify before the United States SenateForeign Affairs Committee about his knowledge of the activities of Major General Edwin Walker, who had been dismissed from the Army over alleged attempts to promote his political beliefs in the military. Lemnitzer approved the plans known as Operation Northwoods in 1962, a proposed plan to discredit the Castro regime and create support for military action against Cuba by staging false flag acts of terrorism and developing "a Communist Cuban terror campaign in the Miami area, in other Florida cities and even in Washington". Lemnitzer presented the plans to Secretary of DefenseRobert McNamara on March 13, 1962. It is unclear how McNamara reacted, but three days later President John F. Kennedy told the general that there was no chance that the US would take military action against Cuba. Within a few months, after the refusal to endorse Operation Northwoods, Lemnitzer was denied another term as JCS chairman. In November 1962, Lemnitzer was appointed as commander of U.S. European Command, and as Supreme Allied Commander Europe of the North Atlantic Treaty Organisation. His time in command saw the Cyprus crisis of 1963–1964 and the withdrawal of NATO forces from France in 1966. As of 2015, Lemnitzer is the only Chairman of the Joint Chiefs to hold another U.S military command after his term as Chairman ended, rather than retiring.