The nature of American warfare greatly changed in the 20th century. One of the most important changes was that American wars of the 20th century were mostly global or international in nature.
Wars such as World War I, World War II, Korean War and Vietnam War involved United States as a global power, and later as a superpower. In many of these wars, United States played a central role and was instrumental in shaping the outcome of the war.
American involvement in World War I was negligible at the start of the war. The country decided to stay neutral when war broke out in 1914 and stayed neutral until 1917. When German submarine attacks led to the deaths of Americans, United States declared war on Germany and joined the Allied forces. Although the American forces were inexperienced, their sheer numbers were important in overwhelming the German forces and bringing World War I to an end.
During World War II, United States emerged as a major world power. It initially decided to stay neutral but the Japanese attack on Pearl Harbor forced US to join the war. The military and industrial might of United States played a very important role during this war. By the end of the war, US military forces were the main pillar of Allied power. United States almost single-handedly brought about the capitulation of Japan while also forcing the surrender of Germany. When the war ended, United States was one of only two superpowers in the world.
Korean War was a cold-war conflict which took place as a part of the indirect confrontation between United States and Soviet Union. Korea became divided between North Korea and South Korea in the first half of the 20th century. In 1950, North Korea attempted a full invasion of South Korea. United States responded by launching a counter-offensive with the support of the United Nations. The bulk of the military resources used in the counter-offensive were provided by the United States. In the end, US was able to push back North Korean forces which were supported by Soviet Union and Russia.
The Vietnam War took place from 1955 to 1975. It was primarily fought between North Vietnam and South Vietnam. However, much of the actual fighting on the South Vietnam side was done by United States forces. North Vietnam, on the other hand, was supported by Soviet Union and its allies. United States used significant resources, personnel and firepower during the Vietnam War. However, the US strategy failed to achieve its objectives. In the end, America was forced to withdraw from the war, South Vietnam was taken over by North Vietnam and Soviet presence in the region grew.
The Persian Gulf War was fought between Iraq and a US-led coalition. The war began when Iraq occupied Kuwait in 1990. The neighboring Arab nations condemned the attack and called upon United States to intervene. United States spearheaded a counter-offensive which relied on the aerial bombing of the Iraqi forces in Kuwait. This quickly led to the withdrawal of Iraqi forces and victory for the coalition.