Sino-Vietnam war
The deteriated of relationship between China and Russia after death of Stalin, and eventually the sino-soviet split. Vietnam gradully become closer with USSR, eventhrough both the russia and China countinued to supply NV. For china, the soviet and Vietnam relationship was a worrying development. After Vietnam war, vietnam became closer to Russia while Cambodia became closer to China. Vietnam invaded Cambodia in late 1978 was strongly encourage by Russia. The soviets made it clear that they were supporting the Vietnam against Cambodian incursions. But China did had different perspective, China saw the distance between Soviet and Vietnam will make USSR difficult to support Viet. China also don't see Vietnam would be important enough for Soviet to go to war.
Dec, 1978, chinese military started to gather along chinese-vietnam border. Chinese military also reinforced chinese-Russia border to prepare for USSR invasion. Jan 28-Feb 4, 1979 Chinese leader DengXP visited USA and warned Vietnam about the possible War.
FEB 17, 1979 China invaded Vietnam. The PRC publicly announced their intention to invade. One of the reason cited for the invasion was the mistreatent of Viet ethnic chinese minority. The goal was to destroy Vietnam military structures along Chinese-Vietnam border, and forced Vietnam out of cambodia which China had failed. Around 80,000 PRC Liberation Army crossed into northern Vietnam. Most of Vietnam best troops were in Cambodia. The Vietnam claimed there are only 100,000 army regular divisions and militiary units in the Northern area. The fighting mainly occurring in the provinces of Cao Bang, Lao Cai, and Lang Son.
March 4, 1979 China took control of Lang son. This was very important because Lang Son was the last defence of Vietnam capital city Ha Noi.
March 5, 1979 China started retreat from Vietnam.
March 16,1979 the war end.
Chinese military- wounded+dead around 26000
Vietnam military- wounded+dead around 39000
Both PRC and Vietnam have different number of casualties, with the Viet claiming that the Chinese suffered 60,000 casualties and 20,000 deaths. The Chinese claim they suffered casualties of about 20,000 to 30,000 with Vietnamese losses at about 40,000 to 60,000.
July 12, 1984 Second Sino-Vietnam War
It was a continuation of the 1979 war, in 1984 PRC decide to take the Laoshan area and surrounding areas. Lao shan is translate as OLD mountain in Chinese. They sucessfully took control without encounter much resistance. Basicly PLA decide to dig in and control the high ground, and Vietnamese force counter attacked.
Vietnam gathered 8 regiments to take Lao Shan which was held by China. It was said the Vietnamese were hiding in the woods before the attack, PLA sent some recon fire from artillary. The Vietnamese captured some positions but was droven back by PLA counter offensive. At the end Vietnamese army redrawed leaving 2000 casualties and approximately 3000 wounded.
PLA rotate troops in and out from Lao Shan, so they could have real combat experience. By 1989 PLA withdraw their troops from Lao shan after a peace treaty was sign.
There are different opinion of who won the war. But this depend objectives of each sides, if the chinese aim was to force Viet withdrawal from Cambodia. The PRC failed while Vietnam have numbers of troops pulled out Cambodia to fight Chinese. Cambodia remained under Viet military occupation for some time. If PRC objective is to punish Viet, the war may have consider as successful. If PRC aims were to test the USSR, then this alliance may have been proven not strong. If China objective was the border disputes, PRC failed to settle it as this war. Bottom line is I think PRC probably had accomplish what they wanted in the war, but the results might not be pretty.