It had also highlighted the importance of well-trained pilots to effective air war. Ultimately, the conflict emphasised the challenges of logistics across often impassable terrain, which generated expensive military campaigns – a feature to reemerge in the postwar period during the conflicts in French Indochina. Thailand's military history in the post-war period was dominated by the growth of Communism across the region, which rapidly became one of the fault lines in the Cold War. Thailand's successive governments found that the Communist bloc in south-east Asia largely consisted of their historical military rivals, and were increasing drawn both into the regional struggle and into having to deal with Communist insurgency at home. Thailand's postwar leaders were mainly traditionalists, seeking to restore the prestige of the monarchy and to defeat the growth of Communism, which was closely associated with Thailand's traditional enemies, the Vietnamese – now in open revolt against the French. Following Thailand's participation in the Korean War, and with the steady growth of US involvement in the region, Thailand formally became a US ally in 1954 with the formation of the Southeast Asia Treaty Organization (SEATO). Whilst the war in Indochina was being fought between the Vietnamese and the French, Thailand – disliking both her old rivals equally – initially refrained from entering the conflict, but once it became a war between the US and the Vietnamese Communists, Thailand committed itself strongly to the US side. Thailand concluded a secret military agreement with the US in 1961, and in 1963 openly allowed the use of their territories as air bases and troop bases for US forces before finally sending her own troops to Vietnam. A Royal Thai Volunteer Regiment (the "Queen's Cobras") and later the Royal Thai Army Expeditionary Division ("Black Panthers"), then a brigade, served in South Vietnam from September 1967 to March 1972. Thailand was however more involved with the secret war and covert operations in Laos from 1964 to 1972. The Vietnamese retaliated by supporting the Communist Party of Thailand's insurgency in various parts of the country. By 1975 relations between Bangkok and Washington had soured; eventually all US military personnel and bases were forced to withdraw and direct Thai involvement in the conflict came to an end. The Communist victory in Vietnam further emboldened the Communist movement within Thailand. After the Thammasat University massacre in 1976 and the repressive policies of Tanin Kraivixien sympathies for the movement increased, and by the late 1970s it was estimated that the movement had about 12,000 armed insurgents, mostly based in the northeast along the Laotian-Khmer border.