In my opinion, the major tactical errors, costing the Axis the war were:
A.) Dunkirk: 140 000 French and 200 000 British soldiers were allowed to escape back to England, forming the bulk force of the allies initially. The Germans could have prevented this, but instead stopped their advances and allowed the rescue to take place. Oh well.....
B.) Battle of Britain: Hitler shifted the Luftwaffe to attack civillian spots, rather than finishing off the RAF, giving the RAF time to organize and form an actual decent resistance. If they would have just taken them out, Europe would have been Hitler's.
C.) Operation Barbarossa: BIGGEST MISTAKE EVER. Those 3 million soldiers Hitler used to invade Russia could have been used elsewhere, and it only dragged Russia into fighting Hitler. This 2 front war is pretty much what ruined Hitler's whole thing. The non-aggression pact was probably the best thing Hitler had going for him, and as soon as he broke it, he pretty much doomed himself.
D.) Pearl Harbor: BIG BIG BIG BIG MISTAKE! What SHOULD have happened was, Germany whipe out Britain and force them to surrender, THEN Germany, Italy, and Japan all crush inward on Russia. I'm pretty sure Russia wouldn't be able to hold off against all three of them. Oh well.
And there's my 2 cents,
-Regards