@moonbus said
Rather a lot depends on which era one takes as a starting point. Pre-WWI maps show countries which ceased to exist after the Soviets occupied Eastern Europe in the aftermath of WWII.
https://www.diercke.com/content/europe-world-war-one-1914-978-3-14-100790-9-36-1-0
Some of those countries, such as Montenegro, reconstituted themselves when the Soviet Union dissolved. Se ...[text shortened]... o say one country or region can declare independence from post-Soviet-era Serbia but another cannot.
Your historiography is a bit inaccurate. In 1914, the greater part of central and Eastern Europe was under the control of three big empires: the Russian, German, and Austro-Hungarian (the Ottoman Empire had already lost almost all its European territory). Montenegro was a rare and eccentric case of a small independent principality / kingdom which existed before World War I and disappeared afterwards. It was after 1918 that many European countries became independent: Czechoslovakia, Poland, Hungary, Estonia, Latvia and Lithuania all emerged out of the collapse of large empires at that time. The Montenegrin assembly itself voted to unite with Serbia in November 1918; a few days later, the union of Serbia and Montenegro was voluntarily joined by the former southern Slavic territories of the Austro-Hungarian empire to form Yugoslavia (officially known, at first, as the Kingdom of Serbs, Croats and Slovenes).
None of this had anything to do with the Soviet Union, which in the post-WWI period had little international influence and had actually lost territory; it was obliged to recognise the independence of Finland and the Baltic States, formerly part of the Russian Empire.
Vastly strengthened after World War II, the Soviet Union was able to impose a Communist system on much of Eastern Europe, but almost all of the countries they controlled remained officially independent. The three Baltic States were incorporated directly into the Soviet Union, but these were the only countries actually to vanish from the map of Europe at that time (they regained their independence in 1991).
Although Yugoslavia also became Communist, it quickly fell out with the Soviet Union and pursued an independent course; so there's no such thing as "post-Soviet-era Serbia". The various constituent republics of Yugoslavia mostly did achieve independence after a bloody civil war in the 1990s. Montenegro, however, remained part of a federation with Serbia (initially still called "Yugoslavia", later "Serbia and Montenegro) until 2006, when it seceded peacefully after a referendum. So it didn't reconstitute itself "when the Soviet Union dissolved" (since it had never been under Soviet control), but fifteen years after the fall of Yugoslav Communism.