re: How will World War 3 start?

It will start in Europe *big surprise*

Europeans will gradually become a minority int heir own homelands and that will create a powder keg.

Eventually some sort of a catalyst will light the fuse. A terrorist attack, an assassination, that sort of thing.

You'll have mini civil wars going on all across Europe as the last vestiges of actual Europe tries to save itself from the cancers they are importing at the moment.