felt that the overthrow of Sihanouk had been one step too far. Many of these royalist officers resigned in protest from the armed forces' structure when Gen. Lon proceeded to transform with American help the old FARK into the FANK to accommodate the character of the new Republican regime. By contrast, new recruits were readily available from the ranks of the far-right Khmer Serei, a US-backed anti-communist guerrilla group led by the hardline Nationalist Son Ngoc Thanh which had fought against Sihanouk's regime during the 1960s and who always viewed him as a communist crony. The measures quickly implemented by Lon Nol's administration included the issue of ultimata demanding North Vietnamese Army (NVA) and Vietcong units to vacate the bases they had established on Khmer soil, and prevented arms shipments bound to the Vietcong from being unloaded at his sea ports. These same measures however, coupled by the effects of the joint US Army/Army of the Republic of Vietnam (ARVN) Cambodian Incursion launched that same year against NVA/VC sanctuaries in Cambodia, resulted in a heavy backlash. In reality, the newly created Khmer Republic and its ill-prepared armed forces were soon caught off-guard in the early 1970s by the aggressive reaction of the NVA, which had previously limited its actions to providing support to Vietcong units operating at South Vietnam even after its devastating defeat in the January 1968 Tet Offensive. The outcome was that the period of Lon Nol's rule actually saw an increase of North Vietnamese military presence in the lower Mekong and Bassac corridors and in the north and northeast Cambodia, particularly from 1972 onwards. In response to the earlier FANK's failed ground offensives to expel them, strong NVA units launched in turn throughout 1971-72 ferocious counter-offensives on these areas – using heavy artillery, tanks, and SA-7 Grail surface-to-air missiles for the first time in Cambodia –, which dwarfed the Tet Offensive numerically. These massive-scale operations only served to exhaust both sides however, and led to the signing in January 1973 of the Paris Peace Accords which marked the official end of American direct involvement on combat operations in Vietnam. The Accords hit both the Khmer Republic and South Vietnam hard, as the military and financial aid that they received from the US was cut by over fifty percent (though American military personnel in Cambodia continued to coordinate the various military aid programs, sometimes finding themselves involved in