For those unfamiliar with what happened in Lithuania, in 1991 Gorbachev used military force to kill 14 Lithuanian civilians who were demonstrating for democratic reforms.
https://www.rferl.org/a/lithuania-soviet-crackdown-1991-krem...
replies(1):
https://www.rferl.org/a/lithuania-soviet-crackdown-1991-krem...
You seem to comment to better inform readers, yet your comment distorts the truth.
Even the article you linked talks about Lithuania declaring independence from the USSR, not asking for democratic reforms.
Despite what your article says, if you read the story on Wikipedia, Lithuania did in fact unilaterally declare independence from the USSR in March 1990.
Just as an example, check what Spain did in 2017 when Catalonia tried to declare independence after a popular vote. If Catalonians decided to resist, there is no doubt that the Spanish state would have used violence to suppress them. Try to imagine what the USA would do if any of its states tried to declare independence.
> Meanwhile USSR constitution had such article. Which was used by Lithuania when declaring independence.
Source?
I can't find any article detailing an exit process in the 1977 USSR constitution. Only at the beginning it says that the USSR is a voluntary union, but that's far from saying it details an exit process.