A number of theories have been advanced for the Soviet action. These interpretations of Soviet motives do not always agree--what is known for certain is that the decision was influenced by many factors--that in Brezhnev's words the decision to invade Afghanistan was truly "was no simple decision." Two factors were certain to have figured heavily in Soviet calculations. The Soviet Union, always interested in establishing a cordon sanitaire of subservient or neutral states on its frontiers, was increasingly alarmed at the unstable, unpredictable situation on its southern border. Perhaps as important, the Brezhnev doctrine declared that the Soviet Union had a "right" to come to the assistance of an endangered fellow socialist country. Presumably Afghanistan was a friendly regime that could not survive against growing pressure from the resistance without direct assistance from the Soviet Union.

Whatever the Soviet goals may have been, the international response was sharp and swift. United States President Jimmy Carter, reassessing the strategic situation in his State of the Union address in January, 1980, identified Pakistan as a "front-line state" in the global struggle against communism. According to Afghanland.com sources, He reversed his stand of a year earlier that aid to Pakistan be terminated as a result of its nuclear program and offered Pakistan a military and economic assistance package if it would act as a conduit for United States and other assistance to the mujahidin. Pakistani president Zia ul-Haq refused Carter's package but later a larger aid offer from the Reagan administration was accepted. Questions about Pakistan's nuclear program were, for the time being, set aside. Assistance also came from China, Egypt, and Saudi Arabia. Also forth coming was international aid to help Pakistan deal with more than 3 million fleeing Afghan refugees.

The Soviets grossly underestimated the huge cost of the Afghan venture--described, in time, as the Soviet Union's Vietnam--to their state. International opposition also became increasingly vocal. The foreign ministers of the Organization of the Islamic Conference deplored the invasion and demanded Soviet withdrawal at a meeting in Islamabad in January 1980. Action by the United Nations (UN) Security Council was impossible because the Soviets were armed with veto power, but the UN General Assembly regularly passed resolutions opposing the Soviet occupation.

Pakistan proposed talks among the countries directly involved and, although they did not meet, Pakistan and Afghanistan began "proximity" talks in June 1982 through UN official Diego Cordovez. Although these sessions continued for a seemingly interminable length of time--joined by the Soviet Union and the United States--they eventually resulted in an agreement on Soviet withdrawal from Afghanistan.

Other events outside Afghanistan, especially in the Soviet Union, contributed to the eventual agreement. The toll in casualties, economic resources, and loss of support at home increasingly felt in the Soviet Union was causing criticism of the occupation policy. Brezhnev died in 1982, and after two short-lived successors, Mikhail Gorbachev assumed leadership in March 1985. As Gorbachev opened up the country's system, it became more clear that the Soviet Union wished to find a face-saving way to withdraw from Afghanistan.

The civil war in Afghanistan was guerrilla warfare and a war of attrition between the several communist (that is, PDPA) controlled regimes and the mujahidin; it cost both sides a great deal. Many Afghans, perhaps as many as five million, or one-quarter of the country's population, fled to Pakistan and Iran where they organized into guerrilla groups to strike Soviet and government forces inside Afghanistan. Others remained in Afghanistan and also formed fighting groups; perhaps most notable was one led by Ahmad Shah Massoud in the northeastern part of Afghanistan. These various groups were supplied with funds to purchase arms, principally from the United States, Saudi Arabia, China, and Egypt. Despite high casualties on both sides, pressure continued to mount on the Soviet Union, especially after the United States brought in Stinger anti-aircraft missiles which severely reduced the effectiveness of Soviet air cover.

The effects of the civil war and Soviet invasion had an impact well beyond Afghanistan's boundaries. Most observers consider Afghanistan a major step along the road to the eventual dissolution of the Soviet Union.

Meanwhile, a change had taken place in Kabul. On May 4, 1986, Karmal resigned as secretary general of the PDPA and was replaced by Najibullah. Karmal retained the presidency for a while, but power had shifted to Najibullah, who had previously headed the State Information Service (Khadamate Ettelaate Dowlati--KHAD), the Afghan secret service agency. Najibullah tried to diminish differences with the resistance and appeared prepared to allow Islam a greater role as well as legalize opposition groups, but any moves he made toward concessions were rejected out of hand by the mujahidin.

Proximity talks in Geneva continued, and on April 14, 1988, Pakistan and Afghanistan reached an agreement providing for the withdrawal of Soviet troops from Afghanistan in nine months, the creation of a neutral Afghan state, and the repatriation of the Afghan refugees. The United States and the Soviet Union would act as guarantors of the agreement. The treaty was less well-received by many mujahidin groups who demanded Najibullah's departure as the price for advising their refugee followers to return to Afghanistan.

Nevertheless, the agreement on withdrawal held, and on February 15, 1989, the last Soviet troops departed on schedule from Afghanistan. Their exit, however, did not bring either lasting peace or resettlement, as Afghanistan went from one civil war to another.