Although communist China had backed North Vietnam in its struggle against South Vietnam and the United States, the Chinese and Vietnamese were traditional enemies; tensions between the two increased when Vietnam strengthened its ties with the Soviet Union, invaded Laos and Cambodia (Kampuchea) in late 1978, and expelled Chinese living in Vietnam. On February 17, 1979, some 120,000 well-equipped Chinese troops crossed the border into northern Vietnam in several places and seized control of several towns; they penetrated 25 miles into Vietnamese territory, encountering stiff resistance. Divisions from Vietnamese occupying forces in Cambodia arrived to reinforce the resistance, which was unable, however, to prevent the Chinese capture of Lang Son, a vital center in Vietnam's northern provinces, on March 3, 1979. About the same time, a separate Chinese force reached the coastal town of Quang Yen, some 100 miles from Hanoi, after several days of fierce fighting against Vietnamese units. Meanwhile, Vietnamese counteroffensives across the border into China's Yunnan province were repulsed. Declaring its punitive military operation against Vietnam a success, China began withdrawing its forces about March 6, 1979, and within two weeks they were all back on Chinese territory. Subsequently, there were many exchanges of fire along the Chinese-Vietnamese border and numerous talks to reach an accord, but no treaty or settlement was concluded.