Many factors can be attributed to the breakup of the Warsaw Pact and the USSR, but the biggest factor bar none is Gorbachev.
When Gorby became leader of the Soviet Union(1) it was basically openly admitted by the Communist Part that the USSR and the Warsaw Pact weren't doing that great. There was no way around that. Grain production was routinely failing to meet quotas, the consumer goods market was still basically stuck in the Stone Age, and the pride of Soviet planners -the heavy industry sector- was languishing from dated technology, few innovations, and tepid growth (4). Although newspapers were de facto prevented from criticizing the Party, they were more than capable -and in fact encouraged- to criticize individuals who the Party believed were improperly carrying out their duties. Soviet citizens learned that when the newspapers published a lot of articles criticizing individual administrators -especially high ranking ones- there were problems in that sector and the newspapers were printing a lot of critical articles.
Gorbachev, in comparison to the geriatric dinosaurs before him, was genuinely relatively popular. He was relatively young, charismatic, and by Soviet standards relatively modern in his outlook. He appeared ready and able to fix the problems in the Soviet Union and make the Soviet Union stronger than America. At the time, many people in the Soviet Union genuinely believed in their Soviet way of life and that it was worth it to put up with some hardship now to secure a better life in the future.
However, pretty much all of Gorby's policies failed to pan out, conditions failed to get better (indeed, they kept getting worse), Chernobyl happened (which was expensive, embarrassing, and a simply a disaster), and the newfound openness in society meant that all the flaws of Gorby's polices were being talked about but there was comparatively little criticism about past policies leading to a rather strong "rose-tinted glasses" effect which further exacerbated the poor showing of Gorby's policies. It goes without saying Gorby's political opponents used full well this new openness to criticize him covertly though the media.
Later on Gorby tried to seize more power for himself by consolidating several leadership offices, installing himself in it, and relegating the former occupants to obscurity. As far as we can tell today, he did this to try and demonstrate to some recalcitrant elites Gorby lead the Soviet Union and hopefully get enough power to fully push through his policies. The problem was, since all of the red tape and conflicts were backroom affairs -and the Soviet elite since Brezhnev had deliberately cultivated the image of always being in agreement- Gorby intent never really got communicated well. This apparent power grab, combined with the problems mentioned above, lead to widespread dissolusionment within the Soviet Union with regards to Gorby. They had put up with the tough time in the hopes things would get better and that Gorby genuinely was trying to make things better, but the lack of improvement and Gorby's apparent willingness to take more power for himself really put a damper on public and elite opinion of Gorby.
Due to the relaxing of censorship, other groups -generally liberal/nationalist- were able to get significant public presence and basically provided an alternative option to Soviet rule. Gorby fundamentally believed that keeping states in the Warsaw Pact/USSR against their will ultimately weakened the Soviet project and, and unlike his predecessors, believed military action should not be used to prop up the hard line governments in Warsaw Pact. Furthermore, Gorby genuinely believed that countries would choose to retain a Communist government/membership in the USSR because Soviet society was, for lack of a better word, better than the west. This might have been true when Gorby first took office, but by the late 80's/90's that was dead in the water. The Soviet monopoly on political discussion was dead and people were sick of centralized Soviet control in their government (2).
Ultimately, it was Gorby's willingness to allow the Soviet Union and the Warsaw Pact to collapse and decision to not take military action/impose martial law that is the most important reason for why the Soviet Union/WP collapsed.
(1) It is important to note that after Brezhnev leadership in the Soviet Union became relatively decentralized amongst a few elite, with these elite often exercising power beyond or less than what their title would suggest. This becomes important later on.
Although Brezhnev's term in office is often described as 're-Stalinization', it was anything but. Stalin's rule was characterized by one man wielding essentially limitless authority and many of the Soviet elite feared that Kruschev was going the same route, securing full power for himself. After the ousting of Kruschev, Brezhnev and his allies took firm steps to ensure that one person couldn't take full power for himself.
(2) Note that leaving the USSR/Warsaw Pact did not mean people were against communism, just that they did not want the USSR telling their government what to do. Remember that the Communist Party remained strong enough in Russia to bring the government into a lockdown until Yeltsin resolved the situation with artillery bombardment, it was only a couple of years ago that Bulgaria elected a non-socialist/communist government, and in Poland Lech Walesa's liberalization/privatization binge was heavily unpopular as he had basically campaigned on a platform similar to Vaclav Havel's "Socialism with a Human Face".
(4) The economic problems facing the Soviet Union at this time were not an isolated issue, the rest of the world was facing some tough economic conditions. The US had just emerged from Stagflation into some rather tepid economic growth and the UK was experiencing the Winter of Discontent.
Were the Korea and Vietnam wars key contributors in the eventual decline of Communism?
Not really. The Sino-Soviet Split became very apparent during Vietnam, and the Soviet Union spent quite a bit of energy fighting with China over who would lead the Third World but it certainly wasn't a main reason for the Soviet collapse.
The economic superiority of the Capitalist West?
Sort of. A lot has been made about how America spent the Soviet Union into oblivion, but it is important to look at who made those statements: former KGB officers and Red Army officers. It fits in well with their narrative that the Soviet system was not undone by its failures, but rather an external force over which they had no control.
Documents in recently opened Soviet archives basically confirm that the 'spending war' between the USSR and America was basically over when Gorby took office.
The 'spending war' certainly didn't help the Soviet Union and funneled money into the economic black hole known as the military but neither would I place it among the main reasons for the collapse.
Pope John Paul II's influence?
Not really. He played a role in Polish nationalism and as a symbol against Communism but as far as I can remember from articles and books on the subject, his influence was mainly confined to Poland as a Polish symbol.
In Poland and many Eastern European countries religious identification is often just a byword for national identification. For example, in Poland and Lithuania a very high percentage of people (IIRC, around 90% in Poland) identify as Catholic, but a much smaller percentage (IIRC around 10%) actually go to church and consider themselves religious.
When I was in Poland as part of a study abroad trip, I had lunch with the Vice Rector of the University JP2 taught at. (He was vice rector during the later years of communist rule.) I asked him about the difference between the number of people identifying as Catholic and the number of people 'being' Catholic. He said that during Communist times the Catholic Church was the only social body that wasn't affiliated with the communist. In some ways it became a symbol of opposition to communism more than a religious institution. (In that way, it becomes ironic considering JP2 clamped down on politically active churches in his time as Pope, openly condemning CELAM.)
JFK, Reagan, other American presidents?
They played a role insofar as they were a warm body leading the opposition to the Soviet Union. Indeed, if it wasn't for Gorby's bending over to Reagan and almost pathetic way he handled foreign relations (3) things could very easily have ended badly. The Soviet leadership genuinely believed that Reagan planned to attack the Soviet Union. (See Able Archer).
Polish Solidarity movement?
Yes, in that they provided an alternative political party to the Soviets in Poland and got some degree of international symbolism although to be honest I'm not sure how well known it was outside of the elites in the rest of the USSR/WP. Pretty much every country had an alternative nationalist/liberal political group challenging the Soviet Union/WP. Even Belarus had one, even though it didn't last very long.
Is China going to remain a communist state indefinitely?
China is communist to the extent that they haven't gotten around removing the hammer and sickle from their icons and how the CCP is intertwined with Chinese modernization.
Why is it working there quite well?
'Communism' is working in China because they are no longer communist in any unique sense, having replaced it with a strong corpratist/authoritarian set of policies.
what about North Korea and Cuba?
North Korea isn't communist, full stop. They purged pretty much every reference to Communism, Marxism, Lenin, and so on from their constitution and state. There is some rhetorical elements and old iconography with communism they haven't quite gotten around to replacing, but the state policy is Juche and only Juche.
(3) As Gorby was loosing support back home, among both the elites and the people, he began engaging in a lot of diplomatic measures to lower tensions between the USSR and the West to a level not seen before even during détente. He was hoping that by gaining a good international reputation he would:
a) Gain prestige/successes to help him weather his crumbling reputation and support back home
b) Make it clear he was someone the west 'could work with' and give them a vested interest in seeing him stay in power
c) Building off of B, hopefully get the USSR better access to international credit markets and better deals for grain imports and technology.
It never really worked out how Gorby wanted. As he became more and more popular abroad as a "peacemaker", he became less and less popular back home in no small part due to him seeming to 'appease' the West in return for no material gain.
EDIT: Whoah! That was longer than I intended.
If anyone is interested in my sources, most of it comes from Vladimir Schlapentoks
The Last Years of the Soviet Empire, a collection of short articles he wrote at the time with a short retrospective on it. A quite interesting book I highly recommend.
