 Complete the following phrase:  As the Soviet Union rapidly withdrew its forces from Central and Southeast Europe, the spillover from the 1989 upheavals began reverberating throughout the Soviet Union itself. Agitation for self-determination led to first Lithuania, and then Estonia, Latvia, and Armenia declaring independence. However, the Soviet central government demanded the revocation of the declarations and threatened military action and economic sanctions. The government even went as far as controversially sending Soviet Army troops to the streets of the Lithuanian capital, Vilnius, to 