Meanwhile, thousands of miles away in the Pacific, the Japanese were preparing to give the U.S. Navy a lesson in applied airpower. It was a lesson the Navy brass should have learned years earlier, when Billie Mitchell, a barnstormer and outspoken advocate of the airplane's military potential, first began needling them about the vulnerability of ships to aerial dive bombing. (The U.S. invented the dive bomber, remember?) The Navy didn't believe their mighty battleships could possibly be threatened by the mosquito-like assaults of little wood-and-canvas crates, so Billy staged a demonstration for them. A captured WWI-vintage battleship of the German Kriegsmarine (Yeah, that's right, long word for Navy. The Germans like long words.) was parked in a convenient spot while Mitchell and his pilots had a go at it with a handful of airplanes. It sank almost immediately. Like most politicians confronted with an inconvenient truth before their very eyes, the Navy admirals simply ignored it, poo-pooing the test results and continuing to build their battleships. Mitchell responded with some very public comments about the thick-headedness of admirals which finally got him court-martialed in 1925. The Japanese were listening, though, and they not only built some dive bombers of their own, but also some of the world's first purpose-built aircraft carriers to put them on. They also built one of the most famous fighter planes in history, the Zero.
The Zero was a pilot's airplane. Like most Japanese vehicles it was lightly-built, agile and cruised forever on a tank of gas. It was a quick and outstandingly maneuverable airplane with a powerful armament, and no other carrier-based plane in service at that time could match it. The U.S. Navy operated the inferior Wildcat, a sturdy but mediocre performer whose only real virtue is that it could absorb a good deal of punishment. The Royal Navy was even worse off, with such lackluster offerings that they eventually just gave up trying to build a decent carrier plane and started buying Grumman Wildcats and converting old Spitfires for carrier duty. The knock-kneed landing gear of the Seafire led to some pretty hairy carrier landings for RNAF pilots, but they were better than nothing.
Zeroes began making a name for themselves over China, where the Japanese had been tearing up the real estate off and on for nearly a decade. When the Japanese began to industrialize, they rapidly learned that their tiny island lacked most of the crucial resources to feed an industrial society. So they decided to take them from their neighbors. Emboldened by the thumping they gave the Russians at the turn of the century and their profitable trip to the conference table after WWI (where their minor efforts against the Central Powers reaped big rewards when the postwar spoils were divvied up), they embarked upon a program of civic improvement which included China and most of the surrounding Pacific Islands. China was an unconquerable wilderness far too vast for Japan to ever hope to completely subdue, but neither could the feuding warlords of China mount an effective defense. There was no such stalemate in the air, however, and Japanese airpower inflicted a great deal of misery upon the Chinese until 1942, when one of China's better connected warlords, Chiang Kai-Shek, begged for help from the U.S.. The U.S. was more than happy to lend them the services of Claire Lee Chennault, an American flier who was just one wrung below Billy Mitchell on the popularity ladder with his Air Force superiors. Chennault was given a handful of P-40 Warhawks and authority to clandestinely recruit some U.S. pilots. Motivated by a $500 a kill bounty and following Chennault's sensible tactical guidance, the Flying Tigers soon became legendary in the eyes of the U.S public and a thorn in the side of the Japanese.
All this fighting made the Japanese war machine very thirsty for oil, and their leaders began to cast a hungry eye towards the oil-rich islands of the South Pacific. When Hitler invaded Russia, the Japanese leadership felt they could count on the distraction to keep Stalin's eyes focused elsewhere while they did some more pillaging. The U.S. attempted to discourage them from further military adventure by placing an embargo on their oil, which is sort of like trying to discourage a starving dog from raiding the trash can by refusing to feed it. War between the U.S. and Japan became inevitable, and on December 7, 1941, the Japanese proved just how prophetic Billy Mitchell's warnings to the Navy had been by sending most of the U.S. Pacific Fleet to the bottom of Pearl Harbor. At more or less the same time, Japanese marines swarmed all over the oil-rich islands of the Dutch East Indies and other strategic islands of the South Pacific. Long-ranged Japanese aircraft made unwelcome appearances all over the Pacific, and things began to look grim indeed for the Allies. The Germans were at the gates of Moscow, Rommel had the British on the run in Africa and U-boats were making jobs with the merchant marine pretty unpopular.
It was at this moment that Hitler, who had thus far chosen to ignore every single treaty he had ever signed, declared war on the U.S. to honor his defense pact with the Japanese. You could probably hear the German generals slapping their foreheads from across the Atlantic.