Why did the Soviet Union invade Afghanistan (in 1978)?

I just watched Charlie Wilson's War and didn't notice if this was explained in the movie.
Why did the Soviet Union invade Afghanistan (in 1978)?
There is a detailed answer in the source, but the short answer the Afghan government repeatedly requested the introduction of Soviet forces in Afghanistan too fight a rebel uprising.
Why did the Soviet Union invade Afghanistan (in 1978)?
1. The Soviets have always been paranoid about their borders. So they've always looked for buffer states to protect them from invasion.





2. The Soviets have also been especially paranoid and contemptuous of moslems.





3. The Soviets originally installed a puppet regime in Afghanistan, assisting in the overthrow of the king and installing the Afghanistan Communist Party.





4. When the Communist party failed to provide enough stability to the country, the Soviet Union was "invited" into the country.





The post that cites Wikipedia and indicates that the government actually asked for the Soviets to come in is a bit disingenuous. First, Wikipedia notes that the post referred to has been challenged as being biased. Second, it is true that the government at the time "invited" the Soviets in. This government had seized power in a coup (the Taraki regime) resulting in the killing of the previous leaders (Daoud regime). They were wildly unpopular as a number of Afghani's allied with Taraki used the secret police to even scores and report people for a variety of false reasons. For most countries, the Taraki regime was seen as a puppet government of the Soviet Union and many other nations refused to recognize that government. The Taraki regime installed a number of extreme decrees: all men were required to shave their beards, Mosques were shut down and no-one was allowed to enter them, under the guise of land reform property and small businesses were confiscated. This was not just a case of islamic fundamentalists being unhappy with the government--most of Kabul found the decrees plus the actions of the govt. and secret police to be intolerable. Nor was this a function of a rebellion that the Taraki or Daoud regimes needed support for. When the Russians seized power and took over the radio station, it involved Russian troops dressed as Afghan soldiers.