The decision to invade had a very clear geopolitical logic, and is one of the lesser moments of Kicking the Dog by the Soviet Union in the Cold War

Oh, certainly. Just to clarify, I'm not saying the USSR were "tricked" or that the decision was in any way irrational. Rather that the Brzezinski saw the Afghan situation developing into one to which invasion would be the rational response and made a conscious decision to help it along.