Serious trouble between Hanoi and the Khmer Rouge under Pol Pot began at the end of the Second Indochina War when both PAVN troops and the Khmer Rouge engaged in "island grabbing" and seizures of each other's territory, chiefly small areas in dispute between Vietnam and Cambodia for decades. What goaded Hanoi to take decisive action was Pol Pot's determination to indoctrinate all Khmer with hatred for Vietnam, thus making Hanoi's goal of eventual Indochinese federation even more difficult to accomplish. Vietnam's Political Bureau had several options in "solving the Pol Pot problem," as it was officially termed. Vietnam's wartime relationship with the Khmer Rouge had been one of domination, in which control had been maintained through the intercession of native Khmers, numbering approximately five thousand, who had lived and trained in North Vietnam. The Political Bureau reasoned that by controlling the Khmer Rouge "five thousand" faction it could control the Khmer (Kampuchean) Communist Party, which in turn would control the Cambodian state and society. This strategy broke down when most of the Khmer communist cadres trained in Vietnam were executed by Pol Pot.

In another effort, the Political Bureau dispatched Le Duan to Phnom Penh soon after the end of the war for a stern meeting with Pol Pot, but his efforts to persuade or intimidate failed. A series of punitive military strikes followed with the objective of triggering the overthrow of Pol Pot. Some of these assaults, such as the one in the Parrot's Beak (see Glossary) region in 1977, involved as many as 90,000 PAVN troops, but they came to nothing. There also were covert Vietnamese attempts to eliminate Pol Pot by bribing his bodyguards to assassinate him.

Finally, in early 1978, Hanoi returned to tested methods of revolutionary guerrilla warfare. Special PAVN teams recruited volunteers for a future Khmer liberation army from Khmer refugee camps in southern Vietnam. About 300 of the most promising were taken to Ho Chi Minh City (formerly Saigon), installed in the former Cambodian embassy building, and organized into armed propaganda teams, with Khmer Rouge defector Heng Samrin in charge of training. The plan, according to program defectors, was to send armed propaganda teams, like the Kampuchea Liberation Front, into Cambodian provinces along the Vietnamese border to infiltrate Khmer villages and begin organization and mobilization work. A Radio Liberation broadcast unit would be established, a liberated area would be proclaimed, and eventually a Provisional Revolutionary Government of Kampuchea would be formed that would then dispatch emissaries abroad in search of support. In late 1978, however, this revolutionary guerrilla war strategy was suddenly abandoned in favor of a full-scale, blitzkrieg-style attack on Cambodia. Later it became evident that the idea for the attack had come from young PAVN officers, many of whom had been trained in Moscow, who had assured the Political Bureau that the matter could be resolved in a maximum of six months. The Political B 1122 Bureau's decision to attempt a military solution in Cambodia was taken against the advice of General Giap and probably most of the other older PAVN generals.

PAVN struck across the Cambodian border from the Parrot's Beak area of Vietnam on Christmas Day 1978. The drive was characterized by a highly visible Soviet-style offensive with tank-led infantry that plunged suddenly across the border, drove to the Thai border, and then fanned out to occupy Cambodia within days. Heng Samrin and his 300 Khmer cadres proceeded to form a new government, called the People's Republic of Kampuchea, in Phnom Penh, and began building an army to take over from the occupying PAVN by 1990. The first indication to the PAVN high command in Hanoi that it was in fact trapped in a protracted conflict came in the summer of 1979, when a major pacification drive, launched by PAVN forces using some 170,000 troops, proved to be inconclusive. It was only in the wake of that drive that PAVN settled down to the slow task of pacifying Cambodia.

Officially, PAVN troops in Cambodia were volunteers, performing what were called their "internationalist duties." The number involved decreased over the years, from 220,000 in January 1979 to 140,000 in January 1987. As the war progressed, Hanoi officials increasingly portrayed it as a struggle against China and labeled the Khmer insurgent forces as Chinese surrogates. By late 1982, they had begun to portray the war as a thing of the past, claiming that Vietnamese dominance had become irreversible, with only mopping up of scattered pockets of opposition yet to accomplish. The Cambodian resistance, however, continued, never able to challenge PAVN seriously, certainly not able to drive it from the country, but still gaining in strength. By 1987 the resistance was stronger than it had been at any time since 1979. To reduce strain on its system and to quiet outside criticism, PAVN lowered the profile of the war. There were fewer military sweeps into guerrilla lairs and greater use of artillery, more static guard duty, and less road patrolling. Military forces concentrated on keeping open the lines of communication, guarding the towns, and building up Phnom Penh's fledgling army--the Khmer People's Revolutionary Armed Forces (KPRAF). At the same time, increments of PAVN forces were withdrawn from Cambodia each year in what the Chinese press labeled the "annual semi-withdrawal performance." By 1986 Hanoi was stating that all PAVN forces would be withdrawn from Cambodia by 1990, a decision officials insisted was "absolute and without conditions."

In retrospect, Vietnam's invasion of Cambodia appears to have been a serious mistake. Apparently it was a decision hastily taken in the belief that a quick, successful takeover would force the Chinese to accept the new situation as a fait accompli. The undertaking was also based on the estimate that Pol Pot had neither the political base nor the military power to resist a traumatic assault, which would shatter his capability to govern and cause the Khmer people to rally overwhelmingly to the new government. Assumptions proved wrong, and the strategy failed. The invasion did not solve the Pol Pot problem, but rather bogged Vietnam down in a costly war that tarnished its image abroad and undermined relations with China that might otherwise have been salvaged. The war drained the economy and continued to be one of Vietnam's unsolved national security problems in late 1987.

Data as of December 1987