The US conduct in Vietnam turned the local population against them, driving many into joining the communists.
Even in 1954, shortly after the end of the First Indochina War against the French, the US was adamant that Vietnam would not fall to communism. When the French left the country, the US ploughed their support behind Ngo Dinh Diem, a Catholic, anti-communist politician who had spent much of the war living in Europe. The US government dispatched CIA agent, Colonel Edward Lansdale to Southern Vietnam with the objective of setting up a pro-American regime in the south. Diem would become a ruthless authoritarian, and under his watch, corruption and nepotism ravaged Southern Vietnam. This was the first of many actions taken by the US government that would alienate the Southern population and turn them away from the Southern Vietnamese government towards the communists.
In the late 1950s, under the close supervision of the US government, Ngo Dinh Diem introduced his agroville policy. In an attempt to stop communist ideology spreading throughout rural Vietnam, he uprooted rural farming communities and forced them to move to fortified government-run farms, called agrovilles. The agroville policy provoked significant resentment from the agricultural communities, dismayed at their removal from ancestral lands. Also, the living conditions in agrovilles were overcrowded. In some cases, an agroville with enough farmland to support around 6,000 farmers, had a population of 14,000 farmers. Food was scarce, and many agricultural workers found themselves unemployed. Rather than preventing rural Vietnam adopting communist ideology, agrovilles helped the spread of it. Later, after US ground forces had arrived in 1965, US military strategy further alienated the local population and drove many more rural communities into supporting the Northern Vietnamese communists. William Westmoreland employed a “search and destroy” strategy for rooting out Vietnamese communists. American troops would search the countryside, identifying Vietcong by searching villagers’ homes for weapons and evidence of insurgent activity. When they found "evidence" they destroyed the enemy. Not only were these missions ineffective, as there was often limited evidence linking any villagers to the communist forces, but the aggressive troops often alienated the local population. This made it easier for the Vietcong to find sympathetic villagers who would hide them, provide them with food and shelter, and even join them in their ambush attacks against American units. In 1968, this came to a crescendo in My Lai. Charlie Company 11th Brigade, under the command of William Calley entered Son My, a small agricultural village in central Vietnam. They encountered no enemy troops. However, according to Calley and Charlie Company, they suspected Vietnamese communists were hiding under the ground in tunnels. Charlie Company 11th Brigade slaughtered more than 500 unarmed villagers, including women and children. Several women were raped, and some of the bodies would later be found mutilated. Events like My Lai, and the general conduct of American troops, meant that US forces never won the hearts and minds of the Vietnamese people. Had the US and South Vietnamese forces won support among the local population, the Northern Vietnamese forces would have found the local population less sympathetic. It would have restricted their assistance from local villagers, rendering their recruitment drive among the Southern population ineffective. It would also have meant fewer local communist informants. Ultimately, it could have changed the course of the entire war.
Douglas Porch and Gian Gentile have attacked the hearts and minds approach in warfare. They argue that it is not an effective counterinsurgency strategy in most cases for a number of reasons. Firstly, a hearts and mind approach, in which the local population are brought on-side to reduce the recruitment pool from which the insurgents can recruit from, leaves the local population vulnerable to attack. The local population become vulnerable to attacks from insurgents. They require constant protection, which requires additional resources. A RAND Study on counterinsurgency, undertaken decades after the Vietnam War, showed that the most effective counter-insurgency strategies are a mixture of an “iron-fist” approach, and one which wins hearts and minds.
If the US Troops had built a friendly relationship with the local South Vietnamese population, they would have rallied against the communism, severely limiting the potency of the Northern Vietnamese guerrilla units, and thereby winning the war.
Enter the technical rejections of the premises here ...