|U.S. military action in Vietnam was a piece in the global Cold War struggle. After Vietnamese nationalists overthrew French colonialists in the 1950s, the country was divided between the Communist north and the anti-Communist south. In the ensuing conflict, Washington backed the south, fearing that a Communist takeover could cascade through Southeast Asia. The first U.S. forces engaged in the conflict in secret, by way of Cambodia. As the civil war intensified in the 1960s, the United States expanded its operations in the region, deploying some 3 million American troops over time, but U.S. forces struggled to gain ground as they fought in difficult and unfamiliar terrain against extremely capable in guerrilla fighters. As the war dragged on and casualties mounted, opposition to the war exploded. By the time American forces withdrew in 1975 and Saigon fell to Ho Chi Minh's Communists, 58,000 Americans and between 1 million and 2 million Vietnamese had died. It was the longest war in U.S. history and the most unpopular American war of the 20th century. In this 1965 photo, paratroopers cross a river in the rain near Ben Cat, in the south.