1) Allies declare war after the Invasion of Chechoslovakia in violation of the munich agreement. Especially if no Tita/Slovakia deal is made.
2) In the event Poland folds over Danzig, the demanding of Kolding (Denmark) or Memel (Lithuania) or Silesia and Posen (Poland), or a straight out attack on Alsace-Lorraine/Luxemburg.
3) A Fascist regime coming to power in Belgium with Nazi assistance.
4) A German supported Italian campaign in Yugoslavia.
A re-occupation of the Rhineland reaction could result in a France/England/Possibly Poland vs Germany war, but it wouldn't be our WWII with USSR, USA, Italy getting involved. It would be a walkover actually.