1991 Soviet coup d'état attempt

The 1991 Soviet coup d'état attempt was a failed attempt made by various leaders of the Soviet Union to take control of the country from Mikhail Gorbachev, who was Soviet President and General Secretary. The leaders formed a group called the State Committee on the State of Emergency, which was pretty much only unified by the belief that Gorbachev's policies were leading to the demise of the USSR. The purpose of the coup was to prevent that from happening. Gorbachev intended to sign a new Union Treaty which would have decentralized power to the republics, which is one of the reasons the coup happened in August 1991 rather than earlier.

The coup's leaders did not really belong to any faction. They mostly owed their positions to Gorbachev, but felt he was leading the country into chaos. There was also an element of self-interest, insofar as there was concern that the demise of the USSR would mean an end to the KGB and other important pillars of the system. Valentin Pavlov, who served as Gorbachev's Prime Minister during most of 1991, seemed to join the coup more because of hearing Gorbachev was going to fire him and due to a desire to override the Russian SFSR government (which he accused of sabotaging the USSR government's economic reform) more than anything else.

The coup failed because it lacked popular support, as most of the population stood at the sidelines rather than take sides for or against it, and the coup further lacked support in the military, where most stood neutral. Press conferences of the coup leaders also showed some of them to be visibly nervous and/or drunk, which failed to inspire confidence and seemed to confirm their ineptness.

In hindsight the coup was a bad idea, regardless of however good the intentions behind it were. It accelerated the USSR's demise in three ways:

1. While Gorbachev was powerless to stop the coup (being holed up in his Crimea vacation resort under house arrest), Boris Yeltsin famously stood on a tank to denounce it, outlawed the CPSU within the Russian SFSR (which was basically tantamount to outlawing the CPSU altogether), and took over Soviet government buildings to place them under the authority of the Russian SFSR government.
2. It made a lot of ordinary citizens believe the only way the CPSU could influence events was via military and KGB rule rather than through persuasion and the validity of its world outlook (and indeed, by 1991 the CPSU was in ideological, organizational, and financial crises that prevented it from carrying out its vanguard role).
3. It encouraged the leaders of the other republics to move toward independence, not only because they saw that the President of the Russian SFSR was more powerful than the President of the USSR, but because the coup attempt implied there might be future efforts to forcibly prevent decentralization and/or independence.