I’m not aware that the Soviet government ever publicly stated objectives for their invasion, leaving us to speculate. Does anyone know more specifically?
If we’re speculating, how about these:
Suppressing a source of incitement of religious fervor in the Central Asian republics, which might have led to open revolt against the Soviet government.
Simple neo-imperial expansionism, perhaps leading toward intimidation of Pakistan and provision of access to a warm-weather port (a Russian goal since Peter the Great).
Providing the military something to do, relieving political pressure on the civilian leadership from military leaders.
Creating a distraction for a disgruntled populace, also relieving political pressure. Declaring an enemy and making war is an ancient political technique, after all.
From what I remember when they first went in, your reason #2 above was a big part of it. Of course, since gaining control of Afghanistan would still leave them without a warm water port (but closer), both Iran and Pakistan had reason to hope they did not succeed (and were thus resistance-friendly).
I don’t know if your reason #1 entered into their calculations, but if it did it certainly seems to have backfired. #'s 3 & 4 were not IMHE factors of great weight. Considering their ongoing face-off with NATO, I doubt the Russian military was really seeking other burdens (and this one became large).
Their claim at the time was that the communist faction then supposedly in charge invited them in to help prop up the faction and prevent it from falling. A hell of a lot like our rationale for being in Vietnam.
Since WWII ended the Soviets wanted “buffer” states that they controlled so they would have plenty of warning that someone was making an aggressive move toward them. Remember, they lost nearly one out of four people in WWII and for them it was an unimaginably bitter experience. So I think that the private Politburo reasoning was that it was really about a buffer zone and keeping Islamic fundamentalism in check.
It was a fools errand and hundreds of thousands died on both sides.
I was not able to find it quickly, but I saw another web page from the last few weeks that outlined the history of British, Russian, and other interventions in Afghanistan from the beginning of the 19th century to the present.
Also, for a formal excuse for the intervention: At the time the already-marxist regime that took over a couple of years earlier was having a crisis – their original government had been overthrown in a violent coup some months before by one of its ministers, who promptly proceeded to lose all control of the internal situation. The Soviet government, seeing the tactic of propping up a “friendly” regime of home-grown Afghani commies fail at securing that southern expansion of their sphere of influence, and concerned because Iran had JUST gone Islamic-revolutionary, decided they’d just have to do it the hard way. They told the faltering Afghani government’s PM that he was going to invite them in to come to his “aid” – and upon arrival promptly shot him and put in his place a more reliable toady, and took to the task of “stabilizing” the place. The rest, you know.
I remember hearing at the time that there was, in fact, an “Islamic Fundamentalist” movement going on in Afghanistan and the Soviets were afraid that it would spread to the predominantly Muslim Soviet Republics in Central Asia. Your reasons 2-4 probably never hurt, either.