Why did aircraft have the stripes on their wings for the invasion of Europe and not before?

The stripes were to identify Allied Aircraft to Allied Anti-Aircraft Units so that the aircraft carrying the Allied Airborne Forces would not be shot down by their own anti-aircraft units as they were at Sicily. The only Airborne operations between Sicily & Normandy was at Salerno. There the entire Fifth Army was simply ordered not to fire their weapons on a particular night. Richard V. Horrell WW 2 Connections