It does raise the question: Would the Soviet Union really go to war with us over a treaty signed prior to the communists even coming to power, and with a country that frankly got themselves into the mess in the first place?
Also, could the threat of nuclear retaliation have kept them out, were they to decide to intervene?
And finally, what part, if any, would their war in Afghanistan (ongoing at the time) have played in their thinking?
Ah, well, we can play "What If?" all day, I suppose. It still remains, however, that faced with acts of war against America, what should a President do? Letting it slide might indeed have been the only viable option available to Reagan at the time, but, with the benefit of hindsight, it was a poor option indeed.