Short answers, pulled from the story.
The Sino-Vietnamese War was a brief armed conflict between China and Vietnam that lasted roughly one month in early 1979. China launched a surprise invasion of northern Vietnam on the 17th of February 1979 and withdrew its troops by the 16th of March 1979, declaring its punitive mission accomplished on the 6th of March.
China cited Vietnam's invasion of Cambodia on the 25th of December 1978, the mistreatment of ethnic Chinese living in Vietnam, and Vietnamese occupation of the Spratly Islands as justifications. Underlying these stated reasons were China's fear of Soviet encirclement and its opposition to the 25-year Soviet-Vietnamese mutual defense treaty signed on the 3rd of November 1978.
Results were mixed. Vietnam did not abandon Cambodia, continuing its occupation until 1989, which China had cited as a primary aim of the campaign. However, China forced Vietnam to redeploy the 2nd Corps from Cambodia to defend Hanoi, demonstrated the limits of Soviet military reach, and strengthened its relationships with ASEAN countries including Thailand and Singapore.
Soviet leaders concluded that direct military support was logistically impossible; the distances were too great, and any reinforcement route would have passed through Chinese-controlled or U.S.-allied territory. Beijing had also promised both Moscow and Washington that the invasion was limited and brief, and when China kept that promise Moscow chose not to retaliate.
Casualty figures are disputed. Leaks from Chinese military sources indicated China suffered 6,954 dead, while Deputy Chief of the General Staff Wu Xiuquan told a French military delegation that China suffered 20,000 casualties overall. Vietnam's state newspaper Nhân Dan claimed more than 10,000 Vietnamese civilian deaths, and China estimated Vietnam lost 57,000 soldiers and 70,000 militia members.
China and Vietnam officially normalized ties in November 1991, following Vietnam's withdrawal from Cambodia and the dissolution of the Soviet Union. The two countries held a secret summit in Chengdu in September 1990 to plan that normalization. A formal border pact was signed in 1999, after many years of negotiations.