I read a lot of tweets from academic and foreign policy types about modern Afghanistan's history and how we still feel them shape and shake current affairs, and a lot are quick to make it sound like the Soviets invaded because they were evil or expansionist or something.

A lot of actual historians and academics relegate Soviet Union decision to intervene to its fear of the spread of radical Islamist ideology in the USSR satellite states. They saw what happened in Iran (Tehran). They saw what happened to the US embassy in Islamabad.

They were aware of Pakistani support of anti-government militia groups (your would-be mujahids), whose training and financial support began as early as 1975 under Bhutto (who had just lost East Pakistan) and ramped up under Zia ul Haq.

We can (and should) disagree about the decision to turn the intervention into a full scale war, but USSR saw what the Americans failed to see - the plague of radicalization that kills Afghan civilians to this day.