When Brejhnev died, he was replaced by Andropov (who died a year after) and Cernenko (who also died a year after) and then a 54 year old guy named Gorbatchev. Mikhaïl noticed how dreadful the state of the Soviet Union was and he promoted "Glasnost and Perestroïka". Glasnost allowed the soviets to open their minds.
It turned against Gorbatchev, but at least, he opened their minds.
Gorbatchev could have kept the soviets in the ignorance and the lies they were told about the western world, as it was the case, before. But no, he opened their minds.
How can Reagan have done this? It had to come from within, from inside the soviet political system. This was the only way to convince the soviets it was the right thing to do.

And of course, when the soviets invaded Afghanistan, many predicted it will be their Vietnam equivalent and it was the case. But the US armed a religious sect called the Taliban....The rest is history.

Raoul