At the onset of World War II, which country had the most powerful navy in the world?

Japanese Navy, 1941

The Imperial Japanese Navy (IJN) at the onset of World War II was the most powerful navy in the world. Japan, like the United Kingdom, was an island nation that was forced to import oil and other raw materials from abroad. This vulnerability impressed on Japan the importance of building a large navy to maintain its sea lanes and secure those resources for itself if necessary.

The IJN was superbly armed and trained. The start of the war saw Japan with ten aircraft carriers, with 1,500 of the best carrier pilots in the world. It had twelve battleships, including the powerful Nagato class, and many modern cruisers, destroyers and submarines. It had the best carrier-based fighter aircraft in the world, the Zero-sen, and excellent torpedo bombers, dive bombers, and even land-based aviation.

At the end of WWII, the U.S. Navy was easily the most powerful navy in the world.