United States involvement, in terms of manpower, was not much in World War I. The US did not declare war until April 1917 and did not start to fully mobilize until late fall of that same year. The United States had been sending money and material to the Allied Powers;...
United States involvement, in terms of manpower, was not much in World War I. The US did not declare war until April 1917 and did not start to fully mobilize until late fall of that same year. The United States had been sending money and material to the Allied Powers; this is partially what led to he United Sates getting into the war, as Germany resumed unrestricted submarine warfare in early 1917 in an attempt to win the war. The major powers of Europe courted American involvement in the war well before 1917, but it was doubtful that the United States would join as most Americans perceived this to be a foreign conflict.
The United States became fully involved in World War II on December 7, 1941, after the attack on Pearl Harbor. Before this, however, the United Sates navy was fighting an undeclared war with the German U-boats who were sinking Allied shipping. The US supplied destroyer escorts to Britain through Lend-Lease. American money, supplies, and food helped to prop up first the British fighting effort and later the Soviet Union effort, as well. American forces played more of a role in this war as the D-Day invasions were largely spearheaded by US troops and much of the fighting in the Pacific was waged by American GIs. The US also developed the weapon that led to the end of the war, the atomic bomb. Unlike the aftermath of WWI, when the United States stood back and let Germany take the brunt of France's wrath in the Versailles Treaty, the US took the lead in rebuilding Europe through the Marshall Plan and the occupying of Western Germany and Japan.
The United States was one of the leading actors in the Cold War. The United States sent troops into Korea and Vietnam in attempts to stop the spread of communism. The US also influenced elections in Italy and Guatemala and stopped a popular uprising in Iran in order to keep pro-Western candidates in power, no matter their human rights records. The US was also the leader of NATO and the main enemy of the Soviet Union; most Western powers followed the US's lead.