Complex sentence: They decided to make massive raids on the German fighter factories; if the Germans chose not to respond they would be at risk of losing the air war without firing a shot, if they did respond, they would meet fighters in the process.
Split into simpler sentences: They decided to make massive raids on the German fighter factories. If the Germans chose not to respond, they would be at risk of losing the air war without firing a shot; if they did respond, they would meet fighters in the process.