See also: Slavophiles.
Pan-Slavism was a movement which commenced in the mid-19th century aimed at a loose unity of all the Slavic peoples. The main impetus came from Russia whose focus was in the Balkans where the South Slavs had been ruled for centuries by other empires: the Byzantine Empire, the Ottoman Empire, Austria-Hungary, etc. Russia saw itself as the "guarantor" and "protector" of all Slavs and played a major role in the liberation of Bulgaria from the Turks.
Following World War II the Soviet Union conquered almost all of central and eastern Europe's Slav countries, with the exception of Yugoslavia, and therefore gained political influence and military control over all those countries until the early 1990s.