To what extent were the American responsible for the overthrow of communism in Eastern Europe ?

and the Soviet Union in 1989-1991

Nothing.

It was their own people that got tired of Communism. They overthrew it.

American had hardly anything to do with the fall of the Berlin Wall.

However, INDIRECTLY, you could argue that the competition to keep up with America in the Arms-Race was one cause to drive the Soviet economy to collapse.

The Americans alone had almost no influence. Western society as a whole was certainly a positive force and an example of the life that may await Eastern Europeans if they ever became free.

The greatest force for the fall of communism was economics and the threat of Soviet reprisals. The communist economies were just not sustainable. Once the USSR was no longer able to support the costs of keeping eastern Europe under control, the outcome was inevitable although it took a couple of decades of decline before it finally happened. Once Gorbachev stated that the USSR would not interfere in Eastern Europe if the countries chose a different path, the communist regimes fell in rapid order. Once the citizens no longer needed to fear a repeat of Budapest in 1956 or Prague in 1968, they took back control of their countries.

It was the combined pressure from the US, the UK, and the Vatican.

Reagan, Thatcher, John Paul II.

Previous rebellions had been crushed ruthlessly by the Soviets, the satellite countries were occupied police state fortresses. Secret police systems that the Gestapo would have envied made sure no-one so much as sneezed without permission.

It was only the collapse of the USSR that allowed the return of freedom to Eastern Europe, and Russia still resents it. I hope, in a way, that Russia re-invades and re-occupies all the old Warsaw Pact nations, just so we Americans can say, Well, you don't believe we had anything to do with your liberation last time, so you can just liberate yourselves entirely on your own this time.

God what asinine revisionism goes on these days.

Poland had already begun to throw off the Soviet shackles when Solidarity over came the national CP. Then the Hungarians left their border with Austria open, and that provided an open door for all the Central European and East Germans to flee. As things got shakier and shakier, the East Germans were desperate for Soviet help to restore control. Gorbachevov declined to intervene, unlike all earlier episodes, chiefly 1956 and 1968. Without the Red Army and KGB backing the puppet governments, they all fell due to popular resistances.

So why did Moscow throw in the towel and let all Stalin's captives go free? That was the result of over forty years of American leadership of the Western powers, American economic and scientific advances, and an economy that caused the Soviet Union to implode .

Those peoples, including all leading Russian dissidents, credit the US, especially the weight of the last straw put upon the Soviet regime when President Reagan called the Soviet Union the "evil empire." That was the moral spark that lit the fires of resistance, having been stoked by decades of Western public discussion of Soviet abuses of human rights.