Short answers, pulled from the story.
Estonia became the first Soviet republic to assert its sovereignty over its own territory in 1989. This bold move triggered a chain reaction across the union by 1990 when fourteen other republics followed suit.
On the 10th of November 1989, the Oblast requested elevation to an Autonomous Soviet Socialist Republic. No legal solution could be found, and the dispute evolved into the South Ossetian War.
Russia, Ukraine, and Belarus signed the Belavezha Accords on the 8th of December 1991. This document officially declared the end of the Soviet Union and established the Commonwealth of Independent States as a replacement framework.
Tatarstan claimed ownership of massive oil reserves and rejected Russian taxes because its population was predominantly Muslim Tatars who demanded control over their resources. The region declared itself an independent state with rights to self-determination in 1990.
Vladimir Putin rose to presidency in 2000 expecting a unified Russia after constituent republics had obtained large amounts of autonomy following the Soviet collapse. His administration passed regulations pushing for reintegration through federal intervention legislation.