Enter the Mustang. Years earlier, the RAF had approached North American aviation about building some P-40's under license. North American replied that it could build a better plane using the same engine in less time. And they did, in an astonishing 178 days. The first Mustangs were great little airplanes, but suffered from lackluster performance above 15,000 feet. The British decided to try dropping a Rolls-Royce Merlin engine into one (the same engine that powered the Spitfire) to see what would happen, and in doing so produced the legendary P-51. It was swiftly adopted as the USAAF's front line fighter plane, although they never did manage to successfully rename it. The Mustang had the range to escort American bombers all the way to Berlin and back, and could match the performance of even the best Luftwaffe fighters. Things were about to get worse for the Luftwaffe.
Things were already getting worse for the Japanese. 1943 saw the debut of two new Navy fighters destined to make the Zero past tense: the Hellcat and the Corsair. The Hellcat looked a lot like the Wildcat on steroids, and it flew like it, too. While still no match for the Zero's maneuverability, it could outpower it, outgun it and outrun it, and was built like a battleship in comparison to most Japanese planes (which lacked self-sealing tanks, pilot armor and really might as well have been built using paper mache for all the protection they offered). The Corsair was even faster. The Japanese, only recently industrialized, lacked the technical capacity to build engines strong enough to power anything much more threatening than the Zero, and with the loss of most of their trained pilots at Midway, were rapidly becoming little more than target practice for the resurgent Allies.
The same could be said of their merchant shipping. Advances in radar and anti-shipping weapons meant that the vital merchant ships the Japanese depended upon to deliver both oil and supplies to and from its far flung island empire were constantly under threat from the air. While the U.S. submarine fleet gets well-deserved credit for putting a stranglehold on Japanese supply lines, better than half of all Japanese shipping losses were due to aerial attack. Enterprising crew chiefs in the Pacific had discovered just how many .50 caliber Brownings you could stuff into the nose of a B-25, and they used them to good effect on the vulnerable supply ships.
Likewise, RAF and American aircraft equipped with anti-submarine equipment were putting an end to the German U-boat scourge. German submarine commanders surfacing in the Atlantic often found themselves immediately wishing they hadn't, as they were invariably pounced upon by allied aircraft and promptly sent to discuss the ethics of unrestricted submarine warfare with Davy Jones. In the Mediterranean, the Americans blind-sided Rommel's Afrika Corps in West Africa and Rommel, caught between the jaws of the Americans in the West and British in the East, could do little but stall for time before the inevitable surrender. Allied air power did what Goering failed to do (at Dunkirk), slamming the door shut on any attempted German evacuation of Tunisia, which left a gaping hole in Axis defenses in southern Europe. The Allies moved to exploit it, rapidly seizing Sicily and invading Italy soon afterwards. Italian shoes have a good reputation for a reason, though, and the boot of Italy proved to be tougher than expected. The Italians themselves were more than happy to surrender almost as soon as the first Allied soldier's stepped ashore, but the invasion bogged down anyway as the more stubborn Wehrmacht put up stiff resistance in the mountains of Northern Italy. However, the Americans did manage to seize vital airfields, which put them in striking range of the Axis oil fields in Romania.