Where did Germany's strategy in World War II fail?

Two examples of the failure of Germany strategy can be seen in the Battle of Britain and the invasion of Russia. In the first, the plan was to use the German Luftwaffe (air force) to gain air superiority over Britain in order to cover an invasion. But the Germans were unable to do so, taking very heavy losses due to the skill of the Royal Air Force and the effective use of radar, among other reasons. Eventually, the Luftwaffe turned to a terror campaign, bombing cities in Great Britain, but this failed as well, as the resolve of the British people did not waver under the almost nightly attacks. So the planned invasion of Great Britain, essential to Hitler's plans in 1941, never materialized, marking a major strategic failure. 

In Russia, Hitler's strategy failed because he underestimated the ability of the Russians to hinder the progress of the German army. When the Germans invaded Russia in the spring of 1941, the assumption was that their attacks would be as swift and devastating as the conquests of Poland, France, and other nations had been. But the Russian Army was able to slow the German advance to the point where it became bogged down in the Russian winter outside the cities of Moscow and Leningrad. The army was completely unprepared for winter fighting, famously neglecting to carry winter gear, and the invasion was a failure. This was a major, and ultimately fatal, strategic blunder because it meant that the Germans were obliged to fight a two-front war once the invasion of France commenced in 1944.