His war tactics, namely Blitzkrieg was first introduced by the brits and adopted for the Wehrmacht by Heinz Guderian who spearheaded the invasions of Poland and France by using the Blitzkrieg doctrine. He was later "demoted" because of disagreeing with Hitlers decision to turn Guderians army south towards Kiev when it was in a position to drive towards the Kremlin during operation Barbarossa and take the capital of the Soviet Union, possibly crippling the Soviet leadership.

Hitler also ordered the armored spearhead driving through France to stop right before Dunkirk, thus giving the British and French troops time to escape to the UK.

Not to mention his obsession with Stalingrad and the ill-advised counterattack in Kursk.

Whatever his art talents may have been, his military decision making was terrible. The successes of the Wehrmacht can be attributed to legendary generals and a huge technological edge over the other participants in the war.

Edit: I do agree that it's wrong to judge a persons intelligence because of the things he has done. Just thought I needed to clear this up about his military strategy skills.