by the US. Thailand had formally become a US ally in 1954 with the formation of the SEATO While the war in Indochina was being fought between the Vietnamese and the French, Thailand (disliking both equally) stayed aloof, but once it became a war between the US and the Vietnamese communists, Thailand committed itself strongly to the US side, concluding a secret agreement with the US in 1961, sending troops to Vietnam and Laos, and allowing the US to use airbases in the east of the country to conduct its bombing war against North Vietnam. The Vietnamese retaliated by supporting the Communist Party of Thailand's insurgency in the north, northeast, and sometimes in the south, where guerrillas co-operated with local discontented Muslims. In the postwar period, Thailand had close relations with the US, which it saw as a protector from communist revolutions in neighbouring countries. The Seventh and Thirteenth US Air Forces were headquartered at Udon Royal Thai Air Force Base. Agent Orange, a herbicide and defoliant chemical used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, was tested by the United States in Thailand during the war in Southeast Asia. Buried drums were uncovered and confirmed to be Agent Orange in 1999. Workers who uncovered the drums fell ill while upgrading the airport near Hua Hin District, 100 km south of Bangkok. US Vietnam-era veterans whose service involved duty on or near the perimeters of military bases in Thailand anytime between 28 February 1961, and 7 May 1975, may have been exposed to herbicides and may qualify for VA benefits. A declassified US Department of Defense report written in 1973 suggests that there was a significant use of herbicides on the fenced-in perimeters of military bases in Thailand to remove foliage that provided cover for enemy forces. Between 1962 and 1965, 350 Thai nationals underwent an eight-month training course in North Vietnam. In the first half of 1965, the rebels smuggled approximately 3,000 US-made weapons and 90,000 rounds of ammunition from Laos. Between 1961 and 1965, insurgents carried out 17 political assassinations. They avoided full scale guerrilla warfare until the summer of 1965, when militants began engaging Thai security forces. A total of 13 clashes were recorded during that period. The second half of 1965 was marked by a further 25 violent incidents, and starting in November 1965, Communist Party of Thailand insurgents