During this period Laos was bombed more heavily than any other country ever has been in history: much infrastructure in the Pathēt Lao zone was destroyed and many were made refugees. The North Vietnamese objectives were more complicated. Their primary goal was to keep the Ho Chi Minh trail in the south open, and to prevent the US using Laos as a base for raids into North Vietnam. The war degenerated into the two sides pushing each other into or out of the Plain of Jars. In 1969 Richard Nixon became President of the US and began the long process of winding down the Vietnam War and finding a political settlement. But this brought no immediate draw-down in Laos. The new administration pursued the same goals by the same means, and in fact during 1969 and 1970 the bombing campaign against the North Vietnamese and Pathet Lao increased in intensity. In the spring of 1969 the North Vietnamese attempted to hold the Plain of Jars through the rainy season. This led to a coordinated campaign that lead to a disastrous defeat of the North Vietnamese. Under constant pressure, their resistance collapsed in the Plain of Jars. They abandoned millions of dollars worth of military equipment and were chased almost to the North Vietnamese border. The success however was short-lived. The North Vietnamese launched a two-division counteroffensive led by a large tank force. All the gains of that year were lost back to the North Vietnamese. In March 1970 the Cambodian government of Lon Nol ended the policy of ignoring the Vietnamese presence in the country. The port of Sihanoukville in Cambodia, which had effectively been a North Vietnamese Army supply terminal for years, was closed by the government. Cambodia soon collapsed into war. This had the effect of making the supply routes from North Vietnam through Laos even more important to the North Vietnamese. In the spring of 1970 the North Vietnamese Army began advancing westward deeper into Laos than ever before. During the same year, units of the Thai Army entered the conflict. These so-called Unity Battalions were in theory volunteers, but were effectively Thai regulars. In 1971 the Royal Lao Army came back into the conflict. The North Vietnamese advance deep into the country destroyed the status quo and prompted the Army back into action. In July the Thai and irregular forces attempted a repeat of the successful 1969 offensive into the Plain of Jars. But the North Vietnamese had learned from their previous mistakes and withdrew in good order ahead of the offensive. While much territory was captured, no serious damage was done to the North Vietnamese Army. The Thai and irregular forces built a chain of fortifications down the middle of the Plain of Jars.