Actually, it is a combination of attrition (winter), lack of air support, heavily fortified positions and battles that grind on forever (not kidding, London and Paris were killing fields). Also, the Royal Navy gave my Navy a pounding, and units were running out of supplies on a regular basis. The fact I was even able to win is in and of itself an major achievement, since I made a few mistakes in my preparations (not enough focus on the airforce)