The Vietnam War was primarily caused by the spread of communism in Southeast Asia and U.S. attempts to contain it. The U.S. supported the anti-communist government in South Vietnam, fearing that a communist victory would lead to the "domino theory," where neighboring countries would also fall to communism. The war had devastating effects, resulting in the loss of millions of lives and significant destruction across Vietnam. It also led to widespread protests and societal upheaval in the U.S., fostering distrust in the government. Ultimately, the war culminated in the withdrawal of U.S. troops and the unification of Vietnam under communist control in 1975.