Underground Yugoslav air base Željava, built during the 50s & 60s at a price of several billion dollars, designed to house 58 fighter aircraft and survive direct nuclear weapons attacks.
Yugoslavia was attacked during the nuclear war on Doomsday, but only Belgrade was destroyed by a 100kt. nuclear warhead, immediately disrupting the chain of command. The state was also badly effected by nearby bombing of neighboring nations. The blast that hit Sofia and Thessaloniki spread radiation into southern Serbia and Macedonia, although the Balkan Mountains and general wind patterns kept these levels to a minimum. Far worse was the fallout blowing across the Adriatic Sea from Italy into coastal Slovenia and Croatia. Generally the north and center of Yugoslavia were mostly unaffected.
Almost immediately martial law was declared, as well as governmental activity being suspended. As refugees flooded in from across the border conflict between these people and the Yugoslavs. After a few weeks it was decided to completely shut down all border crossings as it was decided that the country could no longer support these people. But even with these measures, the countries days had been numbered, as nationalist sentiment in both Croatia and Slovenia had reached an all-time high by late 1985, culminating in Slovenia declaring independence on October 6th, 1985, followed by Croatia two days later.