 Even before the supply conduit through Sihanoukville was shut down , PAVN had begun expanding its logistical system from southeastern Laos ( the Ho Chi Minh trail ) into northeastern Cambodia . PAVN also launched an offensive ( Campaign X ) against the Cambodian army , quickly seizing large portions of the eastern and northeastern parts of the country , isolating and besieging or overrunning a number of Cambodian cities including Kampong Cham . Communist forces then approached within 20 miles ( 32 km ) of the capital , Phnom Penh , spurring President Nixon into action . 
