In 1923 Britain assisted Japan in the development of aircraft carriers. Throughout the 1920s, instructors from Germany trained fledgling Imperial Japanese Navy pilots in obsolescent foreign aircraft. But in the early 1930s, incorporating advanced modern technology, Japan secretly began building its own aircraft. Apparently, shortly before the Pearl Harbor attack, U.S. military strategists still assumed that Japan had only a few hundred relatively obsolete aircraft, copied from older British, German, Italian and American designs.
In reality, at the beginning of the Pacific War, Imperial Japan had the finest naval aviation corps in the world. Japanese aircraft (e.g. the legendary Mitsubishi Zero fighter) were equal or superior to any Western model. Additionally, well-trained Japanese pilots had gained valuable experience since 1937 in the Second Sino-Japanese War.
During the first six months of the Pacific War, Japan achieved a stunning succession of victories. Then came the Battle of Midway where the Imperial Japanese Navy lost >300 pilots and four aircraft carriers in a battle from which they never fully recovered.
Despite Yamamoto’s warning, Japan had no contingency plans for a long war. Most of the operational types with which Imperial Japan began the Pacific War in December 1941, were still front line aircraft when the war ended in August 1945. In contrast, the Allies were able to accelerate the replacement of obsolete aircraft with new improved designs.
At the beginning of the Pacific War, Japan made the decision to deploy all of their most experienced pilots in battle. In contrast, after a brief period of deploying their most experienced pilots very early in the Pacific War, the Americans deliberately retained their best pilots as flight instructors and invested heavily in training facilities. The result was initial Japanese air superiority that soon dwindled with the loss of men and aircraft. Ultimately, Allied aircraft production vastly outpaced Japanese ability to keep up.