It won't matter who fires the first shot or whether an EMP weapon is used because all the major nuclear power countries have weapons at home or in submarines so once the shooting starts, it will be a shitstorm.

Anybody who bombs another country or uses an EMP weapon will very unlikely want to invade it because all the nuclear power plants will go Fukushima and be uninhabitable for ages.

The major powers can do all the dick waving they want but the moment anybody does anything, it's over.

The only real problem would be a maniac that wants to go see (insert religious figure) and wants everybody else to come along.