Though Vietnam has a long history of conflict over its independence from its founding in 208 B.C., U.S. involvement in the affairs of Vietnam began to crystallize during the final years of World War II. At the Potsdam Conference, the Allied powers determined that Britain would occupy Vietnam and force out Japanese troops occupying the area south of the sixteenth parallel. After a summer of internal political unrest in Vietnam, in September 1945, British forces arrived. Though Vietnam had long been a French colonial interest, the Vietnamese resisted French influence and clamored for independence, even attempting to enlist the United States' assistance. In early 1946, the French did assent to recognizing limited Vietnamese independence and Ho Chi Minh as the leader of the Democratic Republic of Vietnam. During that year, the Vietminh attacked French military forces and provoked the French into war, in which the United States supported their French allies throughout the Truman presidency. The French began to reassert their power over Vietnam, but the Chinese and Soviet governments allied themselves with Ho Chi Minh.
Bao Dai, the leader of the French-recognized faction, also claimed that his party, and not Minh's, had authority over the country. By 1950, the Truman administration had begun sending American military advisors to Vietnam to support the French. Eventually, the United States began lending financial support to France's war against Minh supporters. While western nations were outlining such policies as those set forth by the Geneva Convention (1954) and SEATO (1954), internal division within Vietnam continued to escalate. Fearing the threat of the expanse of Communism throughout the Pacific Asian area, the United States, during the Eisenhower and Kennedy administrations, continued aiding the French, until the number of U.S. military personnel deployed to Southeast Asia numbered nearly 20,000. Under the Johnson administration, the U.S. destroyers Maddox and C. Turner Joy received fire from North Vietnamese boats, and President Johnson reacted by ordering an aerial assault of North Vietnam. Only a few days after this incident, Congress passed the Gulf of Tonkin Resolution, which extended to the president the necessary authority to conduct war, though war was never officially declared.
Within months, the first combat-ready unit was deployed to U.S. Marines headquarters at Da Nang in March, 1965. U.S. involvement continued to steadily increase, and by the close of 1967 over a million American troops were in Vietnam, despite the growing sentiment of the American public to stop or withdraw from the war. The undeclared "war" eventually became the United States' longest foreign policy engagement. After years of intense battle, the United States withdrew the last combat troops from Vietnam in March 1973. More than 1.2 million Americans served in the war; nearly 60,000 died in service.
The objective that the United States supported — in short, preventing Vietnam from becoming a communist foothold — was never realized. In April 1975, Saigon surrendered to the communist revolutionaries; the following year, the Socialist Republic of Vietnam was declared.