After several attacks upon them, it was decided that U.S. Air Force bases needed more protection. The South Vietnamese military seemed incapable of providing security. On March 8, 1965, 3,500 United States Marines were dispatched to South Vietnam. This marked the beginning of the American ground war. U.S. public opinion overwhelmingly supported the deployment.

In a statement similar to that made to the French almost two decades earlier, Ho Chi Minh warned that if the Americans "want to make war for twenty years then we shall make war for twenty years. If they want to make peace, we shall make peace and invite them to afternoon tea." As former First Deputy Foreign Minister Tran Quang Co has noted, the primary goal of the war was to reunify Vietnam and secure its independence. The policy of the Democratic Republic of Vietnam (DRV) was not to topple other non-communist governments in South East Asia.

The Marines' assignment was defensive. The initial deployment of 3,500 in March was increased to nearly 200,000 by December. The U.S. military had long been schooled in offensive warfare. Regardless of political policies, U.S. commanders were institutionally and psychologically unsuited to a defensive mission. In December, Army of the Republic of Vietnam (ARVN) forces suffered heavy losses at the Battle of Binh Gia,^[139] in a battle that both sides viewed as a watershed. Previously communist forces had utilized hit-and-run guerrilla tactics, however at Binh Gia they had successfully defeated a strong ARVN force in conventional warfare. Tellingly, South Vietnamese forces were again defeated in June, at the Battle of Dong Xoai.

Desertion rates were increasing, and morale plummeted. General William Westmoreland informed Admiral U.S. Grant Sharp, Jr., commander of U.S. Pacific forces, that the situation was critical. He said, "I am convinced that U.S. troops with their energy, mobility, and firepower can successfully take the fight to the NLF [National Front for the Liberation of South Vietnam]." With this recommendation, Westmoreland was advocating an aggressive departure from America's defensive posture and the sidelining of the South Vietnamese. By ignoring ARVN units, the U.S. commitment became open-ended. Westmoreland outlined a three-point plan to win the war:

• Phase 1. Commitment of U.S. (and other free world) forces necessary to halt the losing trend by the end of 1965.
• Phase 2. U.S. and allied forces mount major offensive actions to seize the initiative to destroy guerrilla and organized enemy forces. This phase would end when the enemy had been worn down, thrown on the defensive, and driven back from major populated areas.
• Phase 3. If the enemy persisted, a period of twelve to eighteen months following Phase 2 would be required for the final destruction of enemy forces remaining in remote base areas.

The plan was approved by Johnson and marked a profound departure from the previous administration's insistence that the government of South Vietnam was responsible for defeating the guerrillas. Westmoreland predicted victory by the end of 1967. Johnson did not, however, communicate this change in strategy to the media. Instead he emphasized continuity. The change in U.S. policy depended on matching the North Vietnamese and the NLF in a contest of attrition and morale. The opponents were locked in a cycle of escalation. The idea that the government of South Vietnam could manage its own affairs was shelved.

The one-year tour of duty deprived units of experienced leadership. As one observer noted "we were not in Vietnam for 10 years, but for one year 10 times." As a result, training programs were shortened.

South Vietnam was inundated with manufactured goods. As Stanley Karnow writes, "the main PX [Post Exchange], located in the Saigon suburb of Cholon, was only slightly smaller than the New York Bloomingdale's." The American buildup transformed the economy and had a profound impact on South Vietnamese society. A huge surge in corruption was witnessed.

Washington encouraged its SEATO allies to contribute troops. Australia, New Zealand, the Republic of Korea, Thailand, and the Philippines^[149] all agreed to send troops. Major allies, however, notably NATO nations Canada and the United Kingdom, declined Washington's troop requests. The U.S. and its allies mounted complex operations, such as operations Masher, Attleboro, Cedar Falls, and Junction City. However, the communist insurgents remained elusive and demonstrated great tactical flexibility.

Meanwhile, the political situation in South Vietnam began to stabilize with the coming to power of Prime Minister Air Marshal Nguyen Cao Ky and figurehead Chief of State, General Nguyễn Văn Thiệu, in mid 1965 at the head of a military junta. This ended a series of coups that had happened more than once a year. In 1967, Thieu became president with Ky as his deputy, after rigged elections. Although they were nominally a civilian government, Ky was supposed to maintain real power through a behind-the-scenes military body. However, Thieu outmanoevred and sidelined Ky by filling the ranks with generals from his faction. Thieu was also accused of murdering Ky loyalists through contrived military accidents. Thieu, mistrustful and indecisive, remained president until 1975, having won a one-man election in 1971.

The Johnson administration employed a "policy of minimum candor" in its dealings with the media. Military information officers sought to manage media coverage by emphasizing stories that portrayed progress in the war. Over time, this policy damaged the public trust in official pronouncements. As the media's coverage of the war and that of the Pentagon diverged, a so-called credibility gap developed.