I think ethnicity and religion are the best answer. During the period when nations are hungry for power, colonizers are exploring the world to find for more sovereignty. Thus, culture and religion vary from place to place which can cause division because of different point of views.
One of the reasons for the division among the nations of Eastern Europe was certainly a strong sense of belonging to a particular entity. Namely, many of the nations of Eastern Europe in some way had a centuries-old tradition of independence and statehood, which they wanted to rebuild and create ethnic states or to reorganize existing by the addition of ethnic territories that they thought belonged to them. There were many overlaps and common interests, which only strengthened the debate and the desire for division. Religion played a major role because it is known that the aspirations of both churches were to influence as many territories as possible, and thus contributed to the strengthening of the sense of ethnicity. It should also bear in mind the "Uniate" church that appears in Ukraine and has a certain impact on Poland. If we add to this the tendency of the communist idea originating from the USSR and its expansion into the countries of Eastern Europe, holding them under steel control, but emphasizing their ethnicity in order to gain the scourge of "independence" and "freedom" that communism offers and the liberation from imperialist and capitalist misconceptions.
Gorbachev's decision to allow elections with a multi-party system and create a presidency for the Soviet Union began a slow process of democratization that eventually destabilized Communist control and contributed to the collapse of the Soviet Union.
