Forty-one years ago! What do we now know, and what evidence do we have? We know that throughout 1979 Kabul (Taraki and Amin) pleaded for Soviet intervention. This was especially the case in March 1979 (during the uprising in Herat). The Soviets turned them down. https://twitter.com/LorenzLuthi/status/1342122863613108227

The evidence is pretty clear on why they turned them down: Moscow was concerned about the fallout from a Soviet intervention for the fortunes of detente (which, though basically on life support, still meant something in Soviet calculations).

What few people know that is that Kabul continued to press for Soviet intervention until about September of 1979 - with the Soviets resisting this prospect. But what Moscow *was* concerned about was the struggle for leadership in the ranks of the PDPA (Afghan Communists).

In particular, they worried that Amin was trying to consolidate power at the expense of Taraki. When the latter stopped over in Moscow on Sept. 10, Brezhnev personally warned him about Amin. "Now I will carefully watch what is happening behind my back," Taraki responded.

But Amin outmanoeuvred his rival, and Taraki soon found himself under arrest (he was later smothered with a pillow in prison). The Soviets first decided to go along with this.

Brezhnev complained at the Politburo on Sept. 20 about Amin's “exceptional lust for power, his brutality in relation to yesterday’s colleagues, his propensity for intrigues and the willfulness of his assessments.”

But he added that "considering the actual situation," the Soviets would just have to work with Amin. What changed this assessment was their suspicion that Amin would sell out to the Americans or do a "Sadat" on them. The Soviets "lost" Egypt in the mid-1970s, and feared a repeat.

So that's why ultimately the decision was taken on December 8 to invade of Afghanistan. The prospect of losing it to the Americans was just too unpalatable. What about the prospects of detente? Well, they must've decided that it was dead anyway.

There is a lot more on this in my forthcoming book - stay tuned.