Austria could destroy NATO. They need to pressure Hungary together with Serbia to leave NATO and form Trilateral Alliance ( TA ). Their combined population is 25,5 million.
Then Austria attacks Czech, Hungary invades Slovakia and Croatia, Serbia together with Republic of Srpska captures Bosnia and Kosovo.
NATO is unable to react effectively since Germany blocks collective response. Poland occupies small territories of Czech and Slovakia near the border but war with Hungary is highly unpopular among Poles.
Polish generals don't want to sent their whole army to the South in case of danger from Kaliningrad Oblast and Belarus.
The whole population of TA rises up to almost 50 million. Germany proclaims neutral status. Some regional governments ( Bavaria ) push to leave NATO.
US are worried about losing Germany and decide to perform air strikes against Austrian army in Czech and Serbian Army in Bosnia and Kosovo from the bases in Germany and Italy.
But their operation only has a very limited success. US understands that they cannot destroy TA forces without ground operation.
Meanwhille TA countries start wide mobilization. Austria occupies Slovenia without significant resistance while main Hungarian forces invade Transylvania and South-West Ukraine.
US pressures Italy and Poland to start ground operation but Italy doesn't want to fight without backing from Germany and France. Those two main European countries are skeptical about starting full-scale war in Europe only to benefit US and UK interests.
Poland is worried about possible strike from Belarus and Kaliningrad Oblast. Polish political and military elites also aren't certain about success of their campaign against such motivated and battle-proven armies like Austrian and Hungarian.
News come that Austria detonates nuclear weapons becoming a tenth country with nuclear weapons. Germany officially leaves NATO. NATO dissolves.
It's a defense union of US, Canada, UK, Australia and NZ now.