Cambodia is one of the eleven nations of Southeast Asia, which also includes: Brunei, Indonesia, Laos, Malaysia, Myanmar, Philippines, Singapore, Thailand, Timor Leste, and Vietnam.
Most of mainland Southeast Asia (now Cambodia, Laos, Myanmar, Thailand, and Vietnam) was at one time part of the vast Khmer Empire. From the 9th to the 14th centuries, the Khmer Empire was concentrated in Angkor in the
northwest of modern-day Cambodia, where Khmer monarchs created massive and elaborate stone temples.
During the 12th century, King Suryavarman II (r. 1113—1150 AD) built the enormous temple known as Angkor Wat, dedicated to the Hindu god, Vishnu. The Khmer Empire reached its zenith in the 13th century under the rule of King Jayavarman VII (r. 1181-1218), who constructed the massive Bayon, built as a shrine dedicated to the Buddha.
In the centuries following Angkor the Empire was repeatedly attacked, with the Thais and the Vietnamese competing for control. By the 19th century, the French had begun to colonize much of mainland Southeast Asia (then called Indochina). In 1863, Cambodia became a French protectorate, and remained under French jurisdiction until 1953.
Throughout the 1950s and 1960s, the U.S. had been at war in Vietnam. In general, the U.S. was staunchly opposed to communism and communist regimes. In particular, they sought to defeat the communist administration based in North Vietnam, first by providing aid and advisors to South Vietnam, and later escalating operations to include U.S. ground troops and air bombings. By 1968, more than 500,000 U.S. troops were fighting
alongside South Vietnamese soldiers facing both a conventional army and guerrilla forces.
In 1969, as part of its efforts to disrupt supply lines supporting the North Vietnamese forces, the U.S. began secretly bombing northern Cambodia (which had officially declared neutrality in the U.S.-Vietnam conflict),where the U.S. believed there were North Vietnamese base camps. Between 1969 and 1973, the U.S. dropped more than 500,000 tons of bombs on Cambodia.