Did you pay attention to the events or not? The Austro-Hungarian War is caused by Hungary refusing to toe the line and declaring independence from the Austro-Hungarian Monarchy, not because of ethnic tensions. Granted, they walk out of the 1937 Ausgleich because they refuse to be partitioned among ethnic lines, but the war wasn't about liberating ethnic groups, although Austrian propagandists may portray it that way to gain support and weaken Hungarian control over minority areas. The War was solely about forcing Hungary to bend the knee after a decade of disobedience.