It is true that for a significant amount of time the US military was quite frustrated attempting to engage the Viet Cong. Attempts to bring them into a conventional pitched battle as demanded by doctrine at the time were completely unsuccessful because they had little fixed infrastructure and were quite effective at hiding among civilian populations. Attempts to interdict flows of foreign weapons, ammunition and military supplies into the country were greatly inhibited by the terrain, the inability of purportedly neutral neighboring countries to secure their own borders, and the specter of Russian or Chinese entry into the war.
However, in the late '60s the US and South Vietnam implemented two rarely discussed programs that together crushed the Viet Cong in the space of one to two years.
1. CORDS, the
Civil Operations and Rural Development Support Program, attached elite US and South Vietnamese military personnel to rural Vietnamese villages. They were given some money, supplies and basic weapons and instructed to organize and train local militias called Regional and Popular Forces ("ruff-puffs"). The intent was that the villages could resist armed Viet Cong groups demanding food and shelter (hostage-taking and collective punishment were common Viet Cong strategy to force compliance from South Vietnamese farming villages that mostly just wanted to stay out of the war). As they increased in number and experience, the ruff-puffs would be organized into a patrol and reconnaissance network that would gradually restrict the areas the VC could operate.
2. The
Phoenix Program, a clandestine and dubiously moral joint effort by the CIA, USSOCOM, Australian SOF and the South Vietnamese secret police, was intended to identify and neutralize supporters of the Viet Cong in the civilian population. Targets were identified by various means, often unethical and unreliable including anonymous tips, hearsay and torture, and then disappeared, with somewhere between a quarter and half of the over 80,000 victims eventually being executed or dying in captivity.
Between CORDS and Phoenix, the ability of the Viet Cong to operate in South Vietnam was completely broken. These programs were so effective that the VC were forced to stake everything on a very long shot - challenging the US and South Vietnamese armies to a conventional set-piece battle. Their Tet Offensive was a complete failure militarily, achieving none of its goals and resulting in such massive losses of men and material for the VC that they were never again able to field an effective combat force.
A few years after US forces were removed from the country, the North Vietnamese army invaded, and defeated the South Vietnamese in a mostly conventional military campaign. The few remaining VC were rolled into the NVA and contributed little.
The fact that the current pop-history narrative of the conflict doesn't really mention this, and in fact tries very hard to convey the opposite impression, is kind of an interesting subject in its own right.
