I’m not asking about what long-term factors inevitably lead to the collapse of the Soviet system; I know there are enough book on the politics and philosophy of the fall of communism, but what hapened in 1991 to topple everything?
Welcome to the SDMB, Drake Tungsten.
I think the Soviet Union collapsed because the United States, for years, forced them into an arms race that they couldn’t’ afford. When Ronald Reagan became president, he upped the ante even more. The USSR simply went broke. 1991 was when that happened. If you’re asking if there was a Great Event in '91 that caused the wall to come down, I don’t think so. Their bank account simply ran out.
PS: Soon, the usual army of SDMB lefties will be along to claim Reagan had nothing to do with it, but that’s my opinion and I’m stickin’ to it! 
How did the US force the USSR into an arms race?
Ronald Reagan had nothing to do with it. The USSR could have just ignored the US military buildup so long as they stayed within their borders. With all their nuclear weapons, the USSR had no worry about the US starting an invasion.
It collapsed because Gorbachev didn’t have the stomach to crush his opposition. Reagan had squat to do with it.
The attempted coup in 1991 was a precipitating factor. Details here:
I think it would have happened anyway, but Reagan’s policies sped it up.
Another major factor was the proliferation of less controllable communications technology (like fax machines) in the USSR.
This Wikipedia article gives you the details, but basically what happened was that the constituent republics that made up the Soviet Union all declared their independance. The Russian government, under Yeltsin, began negotiating directly with other countries. Gorbachev, as Premier of the Soviet Union, no longer had a nation for his government to run.
This happened incidentally in 1991 - three years after Reagan left office.
Well, it certainly wasn’t because of Bush I, who made a point of telling the Lithuanians not to rock the boat as they agitated for independence. But once the Eastern Block shed it’s shackles to the USSR (the dominoes fell fast once the Berlin Wall “came down” in '89), that empire was pretty much done with.
Well, the OP is framing the question so that it’s not answerable, because nothing would have happened in 1991 had it not been for events taking place decades earlier. At the very least the OP needs to understand the impact of the new line introduced by Gorbachev, his push for economic reforms and his call in 1987 and 1988 for democratization. So my short answer is: What Nemo said.
As for the long term factors (the real factors) the key is that the Soviet Union was never one unified country like America. This is why China will not last either. Short story, when Stalin took over the Soviet Union he replaced local governments in the republics with the administrative system used in Russia, and he didn’t hesitate to send people from Russia to the outer republics to see the process through. In other words, Stalin tried to Russificate the republics. But these republics had their own national identities, their own languages and their own culture, and that’s something that’s pretty hard to beat, even if you have 100 years to do the job. I have an execellent book written in the mid-70’s which lists the demographics of the population in every major Soviet republic, detailing how many still speak their old language even though they should speak Russian and many other things, and explaining how these differences in national identity eventually will lead to the downfall of Soviet.
I think Reagan had little to do with it, any contribution was the opposite of what John Carter of Mars thinks. I think Reagens greatest contribution was that he actually believed something could be done to change things. Mikhail Gorbachev was a brilliant leader, he understood that the Soviet Union couldn’t be held together by military might alone, because you need national identity to keep a nation together. Though I think he truly believed he could hold Soviet together as one nation through the transformation, which he couldn’t. Another person who played, IMO, an important role for a short period was Lech Walesa, as an inspiration for the Baltic Republics 10 years later.
As for Reagan, I find it kind of ironic that so many believe that Reagan forced the Soviets into an arms race. It’s true that Reagan was trying to strengthen NATO (or modernize might be a better word), but simultaneously he was working for arms reduction. In a major statement from the White House in November 1981, Reagan proposed two things: Renaming the negotiations on strategic nuclear weapons from SALT (Strategic Arms Limitation Talks) to START (Strategic Arms Reduction Talks), and the “zero option” (a German creature) which was the idea that neither side should have intermediate-range nuclear forces in Europe. Later on Reagan expanded on this to apply globally. Reagan believed, apparently, that the Soviets couldn’t afford to say no because they couldn’t afford an arms race to catch up with the strategic advantages of the neutron bomb. He might have been right. But the thing is: The Soviets didn’t say no, so there were no real arms race.
Here’s the fascinating transcript from the famous Gorbachev - Reagan meeting in 1986:
http://www.cnn.com/SPECIALS/cold.war/episodes/22/documents/reykjavik/
Here’s an interview with former President GHW Bush on this:
http://www.gwu.edu/~nsarchiv/coldwar/interviews/episode-23/bush1.html
And here’s another interesting link I found when googling for the timeline:
http://www.findarticles.com/p/articles/mi_m2751/is_79/ai_n13504410
They would have had to develop a counter for the Space Defense Initiative, which Reagan was serious about developing, or fold their hand. They folded.
Do you deny that there was an (expensive) US vs. USSR arms race from the end of WW2 until, at least, the late '80s?
Or launch a pre-emptive nuclear war. Which now open Soviet archives have shown was given serious consideration during Reagan’s presidency.
I think many factors led to the demise of the USSR (IMHO Gorbechev’s pro democracy tolerance was the biggest reason).
I think Reagan’s military “build up” may have had an impact, but to say that he won the Cold War is either moronic or just plain Republican bias.
It seems to me that the OP is asking, “What was the straw that broke the camel’s back?” (The failure of the Red Army in Afghanistan, Margaret Thatcher’s liberation of the Falklands, Ronald Reagan’s firm-mindedness, the arms race, Pope John Paul II, all were major factors)
I think the coup attempt and its aftermath are good candidates for the proverbial Straw That Broke The Camel’s Back. When the Berlin Wall fell in 1989, the USSR lost control over its satellite states. When some of the Soviet republics (Lithuania has already been mentioned) began agitating for independence, the USSR lost control over its outlying regions. The coup showed that the Communist Party no longer had control over itself, and that was the worst. The Party was the government - if it was fragmented, how could it control such a huge country? And when the coup makers commanded the army to use lethal force to stop the protests, and the ordinary man-in-the-tank refused to shoot, it showed that the government no longer had the absolute loyalty of the military. What was left?
And with Gorbachev imprisoned, it was Yeltsin who became the hero of the day. (Note to younger Dopers: This was when he was a functioning alcoholic.) Yeltsin was head of the Russian Republic, and not part of the Soviet government. When even Mother Russia was asserting herself as an independent country, it was Game Over. The Soviet Union limped along for a few more months, but the fatal blow had been struck.
And for the nostalgia buffs out there, how about a reminder of the funniest joke of autumn 1991?:
Q: What’s the difference between the USA and the USSR?
A: The USA has a communist party. 
Historically low oil prices also contributed to the fall of the U.S.S.R. They were going broke.
According to an article I read once in The Atlantic Monthly, the U.S.S.R. didn’t increase its military spending in response to SDI.
Korea. After that there was a massive buildup in Eastern Europe and from there the race for superiority continued.
Marc
Back then the Soviets claimed that they had a different approach to counter SDI. (And today it looks like they didn’t lie, though the facts are unclear, military secrets and all that).
In the CNN-transcript I linked to above it seems that Gorbachev isn’t to concern about SDI. He says: “As far as SDI is concerned, it is not evoking concern among us today in the military respect” and “We will not deploy SDI. We have another concept.” FWIW.
John Carter of Mars, it’s quite a statement to claim that SDI was important to the fall of the empire. Do you have a cite from anybody at the Soviet side who have come forward now 20 years later to confirm this?
Here’s a cite from my initial post (#2) in this thread.
Not to be smarmy, but that should indicate that I’m not willing to spend hours searching the 'net to prove that there was, indeed, a cold war and an expensive arms race associated with it…
China has been a civilization for thousands of years. It has not always been Communist, but it has always been China, even after the Mongols took over. The people think of themselves as Chinese, as a people.