No Country Emerged from the Union of Serbia, Montenegro & Austria-Hungary

The premise of the question contains a historical misunderstanding. No single country was formed from the breakup of a union between Serbia, Montenegro, and Austria-Hungary. These were three distinct political entities with different fates:

• Austria-Hungary was a vast dual monarchy that **collapsed entirely in 1918** at the end of WWI. Its territory was carved into new states: Czechoslovakia, Yugoslavia (initially Kingdom of Serbs, Croats, and Slovenes), Poland, Romania, Italy, Austria, and Hungary.
• Serbia and Montenegro were **independent kingdoms** that fought on the Allied side. In 1918, they **voluntarily united** with former Austro-Hungarian South Slav lands (Slovenia, Croatia, Bosnia, Vojvodina) to form the **Kingdom of Serbs, Croats, and Slovenes (renamed Yugoslavia in 1929)**.

So, Yugoslavia emerged from the **union of Serbia and Montenegro *with* South Slav territories *from* Austria-Hungary**—not from a three-way union of the three named entities. Austria-Hungary ceased to exist; its pieces joined various new nations.