not reading through all the replies, so could have been said already, but i got a bunch:
1-the failure of the french at Agincourt to have any real leadership, which lead to them not using their crossbows, and, especially, not charging at the english while they advanced across the field into arrow range, while they where disorganized and the archers didnt have their stakes placed in the ground o protect from cavalry charges.
2-hitlers changing tactics in the battle of britian to the blitz, when the RAF was nearly destroyed. (for those that dont know, a german pilot jettisoned bombs on london to speed his craft, causing churchill to order retaliatory raids on berlin, causing hitler to order the blitz). if he had destroyed the RAF, he could have destroyed the royal navy. if he destroyed the royal navy, he could have invaded britian and won the war by being able to take russia 1v1, the USA not having a base in europe.
3-hitlers declaration of war on America. I dont care if he signed a treaty with japan, japan broke the treaty by not declaring war on the soviets (the treaty stated the japanese and germans would come to each others aid in war with america or russia. Japanese didn't declare war on soviets, or history could be very different, the elite siberian ski troops not being able to save moscow from the germans in winter of 41-42). if you argue that the japanese had signed a treaty with soviet uniion, what did the axis care for treaties????? hitler had already broken the ones hed signed with poland and russia.
4-this one is personal. it really didnt have a major impact on history, but it really rankles me cause of the idiocy. At midway, the USA had three carriers, the hornet, yorktown, and enterprise. one of the USA's most elite torpedoe squadrons, with probably the best torpedoe skipper in naval history (at least the best ive ever read about) was on the hornet. some ***** in the hornets high command decided to launch the fghters first, and use up all of their limited fuel circling the carrier, waiting for the bombers to form up, and someone (probably the same person) decided to make the bombers wait half an hour to take off. several people had calculated the correct position of the japanese fleet, but the same idiot sent them on a completely wrong course. the torpedoe bomebrs had no fighter protection to being with, the idiot thinking it best to keep them all with the dive bombers, despite evidense at the coral sea that pointed to the contrary.when the torpedoe skipper decided to pursue the correct course to the japanese fleet, the fighters continued with the dive bombers, and later crashed due to lack of fuel on the way back. well, the torpedoe bomebrs reached the japanese fleet, but not a single torp hit, the obsolete machines recked by japanese fighters. USA lost its best torpedoe captian there. had the idiot on the hornet used his aircraft correctly, the yorktown could have been saved, as the horents aircraft could have sunk/damaged the japanese fleet, preventing the yourktown from ahving to sail at half speed or whatever into the location it got sunk by the submarine.
needless ot say, the idiot was never put in charge of aircraft again. you canr ead all about it in a book called A Dawn Like THunder