Germany's greatest mistake that cost them the war

Lert: I don't think they could've won anyways.   However, not invading russia and not diverting their bombing efforts from britain's airfields to London (but making 110% sure the RAF was destroyed instead, thereby 'winning' britain) might've lasted a few years longer.   1) Invading Russia cost them way too much 2) The US would've had a very hard time launching a proper invasion of Yurp without Britain as a springboard.