Soviet Invasion of Afghanistan 1979

Christmas 1979. Soviet armor pours across the Afghan border towards Kabul as helicopters secure the mountain passes through the Hindukush mountains. In Moscow, the Politburo has decided to save Afghanistan’s communist government from collapse. Afghan rebels have taken up arms against the unpopular regime and control most of the countryside. But the Red Army leadership doubts it can pacify the country - so why did the Soviet Union invade Afghanistan?