Here's an old answer from this sub to a related question from /u/CptBuck. : https://www.reddit.com/r/AskHistorians/comments/hjcxmi/the_soviets_invaded_afghanistan_in_1979_because/fwm6sxq/

I don't know the answer to your question, but it might be helpful to note. The USSR wasn't fighting a war against Afghanistan, they were helping the communist government fight a civil war/uprising at the (unpopular) governments request.