I seem to remember, in the aftermath of the Soviets invading Afghanistan, there was a lot of speculation by the talking-heads as to why? And the story that stuck in my mind was that the Soviets were trying to annex a corridor to the Indian Ocean because it was too easy to bottle up their fleet otherwise. I heard that theory again when the US aiding Pakistan was being discussed.
Was there ever really a serious plan by the USSR to invade all the way through Pakistan to the Indian Ocean? Or was that story just a plausible enough sounding talking point to rationalize US involvement? Or was it Soviet propaganda to rationalize their invasion really carried out for other purposes? Or was it just complete BS dreamed up by talking-heads?
I've never seen a credible historian suggest that the Soviets were interested in an Indian Ocean Port (or thought that they could secure a supply route through the treacherous mountains of Afghanistan). At least not in the time frame the war was going on. Like Vietnam, Afghanistan was a prestige war. The Soviets had seen their influence in the Middle-East waning in the late 1970s. The uprising against the Communist government in Afghanistan would have meant a further reduction in that influence and their over-all prestige. Intervention was to both demonstrate Soviet resolve in supporting friendly governments, as well as to rescue their increasingly poor situation in Central Asia and the Mid-East.