Serbia has been building up its military and has declared its intention to unite with Serbs living in other countries while also taking an authoritarian turn and having friendly relations with Russia and China.
Eastern European countries were brought and kept in the Warsaw pact by force. These countries wanted to join NATO because they were afraid Russia would invade them.
Before many of them joined NATO, Russia was already violently intervening in its neighbors.
Poland threatened to develop nuclear weapons to defend itself, and got involved in U.S. politics to push for it joining NATO.