There was little sign of improved relations with the Soviet Union, and Deng Xiaoping declared the 1950 Sino-Soviet friendship treaty to be null and void. Relations with Vietnam suddenly turned hostile in 1979. These two countries had once been allies, but in 1972 Mao Zedong told Vietnamese premier Pham Van Dong that they should stop expecting Chinese aid and that the old historical feud between the two countries would erupt again. China also gave its support to the genocidal Khmer Rouge regime that took power in Cambodia during 1975, provoking the hostility of the Soviet-backed Vietnamese government. In January 1979, Vietnam invaded Cambodia and drove the Khmer Rouge from power. During his U.S. visit, Deng Xiaoping remarked that Vietnam would have to "be taught a lesson". In February, a full-scale Chinese attack was launched on the Vietnamese border. Although China withdrew after three weeks and declared its objectives met, the war had not gone well and demonstrated the country's weakness. The PLA lost over 20,000 men due to weapons and equipment that were outdated, poorly made, and in short supply (a side effect of the disruption caused by the Cultural Revolution), maps that were decades old, the continued use of human-wave tactics from the Korean War, and the fact that the army had no system of ranks. Even worse was the fact that the Chinese had not even engaged Vietnam's regular army (which was in Cambodia), but instead fought militia and home guard units. On the other hand, it was a political victory since the Soviet Union had failed to come to its ally's aid and contented itself with verbal protests. In reference to Moscow's inaction, Deng Xiaoping reiterated Mao Zedong's saying that "You can't know how the tiger will react until you poke its backside."