On returning to Cambodia, in October Pol Pot ordered the country's army to switch tactics, adopting a defensive strategy involving the heavy use of land mines to stop Vietnamese incursions. He also cautioned the army to avoid direct confrontations which would incur heavy losses and instead adopt guerrilla tactics. In November 1978, the CPK held its Fifth Congress. Here, Mok was appointed the third ranked figure in the government, behind Pol Pot and Nuon Chea. Soon after the Congress, two senior government members—Vorn Vet and Kong Sophal—were arrested and sent to S-21. This precipitated another round of purges. Vietnamese Invasion: 1978–1979 On 25 December 1978, the Vietnamese Army launched its full-scale invasion. Its columns initially advanced into north-east Cambodia, taking Kratie on 30 December and Stung Treng on 3 January. The Vietnamese main force then entered Cambodia on 1 January 1979, heading along Highways one and seven toward Phnom Penh. Cambodia's forward defences failed to stop them. With an attack on Phnom Penh imminent, in January Pol Pot ordered Sihanouk and his family to be sent to Thailand. The entire diplomatic corps followed shortly after. On 7 January, Pol Pot and other senior government figures left the city and drove to Pursat. They spent two days there before moving on to Battambang. After the Khmer Rouge evacuated Phnom Penh, Mok was the only senior government figure left in the city, tasked with overseeing its defence. Nuon Chear ordered the cadres in control of S-21 to kill all remaining inmates prior to it being captured by the Vietnamese. However, the troops guarding the city were unaware how close the Vietnamese Army actually were; the government had concealed the extent of the Vietnamese gains from the population. As the Vietnamese approached, many officers and other soldiers guarding the city fled; the defence was highly disorganised. There were isolated examples of Cambodian villagers killing Khmer Rouge officials in revenge. In January, Vietnam installed a new government under Samrin, composed of Khmer Rouge who had fled to Vietnam to avoid the purges. The new government renamed Cambodia the "People's Republic of Kampuchea". Although many Cambodians had initially hailed the Vietnamese as saviours, over time resentment against the occupying force grew. The Khmer Rouge turned to China for support against the invasion. Sary travelled to China via Thailand. There, Deng urged the Khmer Rouge to continue a guerrilla war against the Vietnamese and to establish