On February 17, 1979, a massive 30-minute artillery barrage rocked the China-Vietnam border. They were the first of 880,000 shells that China's People's Liberation Army (PLA) would fire at its neighbor over the next three and a half weeks.

Within hours, some 200,000 Chinese soldiers crossed the border into Vietnam. They were supported by an additional 400,000 troops, hundreds of tanks, and 7,000 artillery pieces.

Their mission was to seize provincial capitals and obliterate any Vietnamese Army (PVA) forces in the areas between them. Despite initial breakthroughs, progress slowed, and the PLA found itself bogged down in a costly war in which it drastically underperformed.

Launched to "teach Vietnam a lesson," as Chinese leader Deng Xiaoping claimed, the invasion was China's first large-scale military action since the Korean War in 1953, and it remains the PLA's last full-scale war to this day.

The invasion surprised some in the West because China had been a steadfast supporter of Vietnam during its wars with France and the US. More than 300,000 PLA troops served in Vietnam between 1965 and 1969, with some 1,100 killed and 4,300 wounded. China also sent billions in aid to their communist brethren.

But tensions between the two communist "brothers" had been boiling for decades. Chinese domination in previous centuries left a general distrust of China in Vietnam, and border battles between China and the Soviet Union in 1969, during the Sino-Soviet split, made it clear to Vietnam that it would soon have to pick between its two benefactors.

Vietnam also faced rising tension and increasing border clashes with the murderous China-backed Khmer Rouge regime in neighboring Cambodia. That, along with Beijing's reluctance to send more aid to Hanoi, led Vietnam to side with the Soviets.

On November 3, 1978, Vietnam signed the Treaty of Friendship and Cooperation with the USSR. It wasn't an outright mutual-defense treaty, but it did include some security promises. Tensions escalated to the point where up to 150,000 Chinese living in Vietnam left for China.