The Vietnam War was a long, costly and divisive conflict that pitted the communist government of North Vietnam against South Vietnam and its principal ally, the United States. The conflict was intensified by the ongoing Cold War between the United States and the Soviet Union. More than 3 million people were killed in the Vietnam War, and more than half of the dead were Vietnamese civilians. Opposition to the war bitterly divided Americans and Australians. Communist forces ended the war by seizing control of South Vietnam in 1975, and the country was unified as the Socialist Republic of Vietnam the following year.