On December 27, 1979, the Soviet Union began military operations in Afghanistan.  Through the lens of the Cold War, western observers viewed this invasion of Afghanistan as a typical example of Russian expansionism and aggression, akin to the Imperial Russian take-over of the neighboring Central Asian republics or the Soviet seizure of Eastern Europe. 

American and western European analysts at the time cited the Russians' centuries-old desire for warm-water ports, suspecting that Afghanistan was merely a stepping stone for Soviet expansion into Pakistan or Iran.

  Others noted the Islamic Revolution in Iran the same year, and theorized that the Soviets wanted to seize Afghanistan to prevent Islamist revolution from spreading across the border into the predominantly Muslim Soviet republics of Central Asia.

In fact, however, the Soviets had no intention of occupying Afghanistan, a friendly client state for the previous 60 years.  They merely sought to prop up communist allies in the Kabul government, but ended up being dragged step by step into a decade-long war.

Lead-up to the Invasion:

A communist coup in April of 1978, the Saur Revolution, toppled the king's government in Kabul, but the Afghan communist party was divided into two viciously feuding factions.  Instead of consolidating power and working to win over the hearts and minds of the Afghan people, the new communist leadership under Nur Muhammad Taraki spent its energy purging and murdering thousands of its fellow communists from the other faction.  This infighting did little to recommend communism to ordinary Afghans, devout Muslims who were unlikely to readily embrace an alien, atheist ideology in any case.

The Soviet government under 73-year-old Leonid Brezhnev and other elderly members of the Politburo strenuously advised Taraki's government to stop the purges and reach out to the Afghan people.  Brezhnev's representatives even counseled the Afghan communists to tone down their anti-Muslim rhetoric in consideration of the vast majority of their subjects' beliefs.  This had no effect, however.  Instead, splinters formed within each of the two communist factions in Afghanistan, leading to further violence.

Taraki's reforms, such as redistributing wealthy land-owners' property to the poor and establishing schools for girls throughout the countryside, had also sparked widespread protest from traditionalists in the countryside.  As events spun out of control, Taraki sent repeated requests to Moscow for Soviet troops to intervene.  The Soviets refused to get pulled in; KGB head Yuri Andropov noted that Soviet troops would only end up fighting and killing the people of Afghanistan, the very population that was supposed to benefit from communist reforms.

In September of 1979, Deputy Prime Minister Hafizullah Amin ousted President Taraki, although they were theoretically from the same faction.  Amin had Taraki assassinated and began a new round of bloody purges, killing thousands of Taraki's allies and loyalists.  When the Soviets protested, Amin replied, "Comrade Stalin showed us how to build socialism in a backward country: it's painful to begin with, but afterwards everything turns out just fine."

Things were anything but fine in Afghanistan in late 1979, however.  Civil war broke out all over the country, with communists battling one another and Islamists battling communists.  Soviet civilian and military advisers, of whom there were perhaps 2,500 in the Afghanistan at the time, were targeted and several hundred died.

As the country descended into chaos, Amin sent increasingly frantic requests to Moscow for Soviet troops to intervene.  His government controlled only 20% of the country, and was losing ground.  The Soviet leadership was still undecided, well aware that any intervention could turn into a Vietnam-style war of attrition.  Now Yuri Andropov of the KGB changed his mind, pushing for an invasion; he had promised to protect Taraki, and felt humiliated by his failure to do so. 

Andropov advocated ousting Amin and replacing him with the more tractable Babrak Kamal.  The leaders of the Spetznatz, the military special forces, also wanted to take out Amin.  Other Soviet officials in Kabul urged the government to intervene and stabilize the situation while working with Amin, who was brutal but loyal to the USSR and to communism.  The pro-invasion leaders felt that their troops could get in, calm the situation, and get out again in six months.

All In:

On December 27, 1979, Soviet special forces with KGB leadership stormed the presidential palace in Kabul, killing President Amin and his family.  They installed Babrak Kamal as his replacement. 

Meanwhile, four Soviet Army divisions moved into Afghanistan, one landing at Bagram Air Base, and three more rolling into the country from the Turkmen Soviet Socialist Republic (now Turkmenistan).  They were later reinforced by two additional divisions, for a total of more than 100,000 Soviet troops on the ground.  In the end, as so many had predicted, this six-month stabilization project turned into a disastrous, unwinnable, nearly ten year long war against the people of Afghanistan - a war that the Soviet leadership never intended to start.

For a fascinating account of these events, based on Russian sources, see Rodric Braithwaite's book Afgantsy, published in 2011 by the Oxford University Press.