South Korea, then ruled by a military leader, sent some 320,000 troops to fight alongside the U.S. in Vietnam, the largest contingent of any U.S. ally.

On Feb. 12, 1968, South Korean marines entered the village of Phong Nhi in South Vietnam's Quang Nam province. It was less than two weeks into the Tet offensive, launched by North Vietnamese and Viet Cong troops, and the marines were looking for Viet Cong fighters.

The U.S. paid Seoul some $5 billion in wages and assistance from 1965 to 1973. U.S. military procurement, meanwhile, helped South Korean conglomerates including Hyundai grow into industrial giants.