The sinking of the Lusitania by a German U-boat in 1915 and the Zimmerman Telegram that attempted to incite a Mexican invasion of the USA. These two things convinced the American public of the need to enter the war and allowed president W.Wilson to do it.
WWII:
The USA remained neutral for two years, but the surprise Japanese attack on Pearl Harbor of December 7, 1941, outraged the public. Congress authorized president FDR to declare war on Japan.
Korea:
It was already the Cold War. Washington thought that the invasion of South Korea by North Korea in 1950 was an elaborated plan to expand communist and it reacted to oppose and impede it. It was the rationale of containment.
Vietnam:
Same reason as in Korea: to stop communist aggression and expansion in Asia, following the containment doctrine.
Between 70,000 and 100,000 years ago, Homo sapiens began migrating from the African continent and populating parts of Europe and Asia. They reached the Australian continent in canoes sometime between 35,000 and 65,000 years ago.
