The plan considered Britain, Russia, France, Germany, and the United States while rejecting an impossible requirement to defeat every coalition. The United States still debated how much navy Congress wished to maintain, but it remained part of the calculation because an expedition to Japan would draw strength away from the Atlantic. Planners judged that a likely European force might centre on four British or Russian battleships reinforced by two more from another power. Six modern Japanese battleships would therefore force an opponent to strip strength from other stations or accept serious risk in East Asia. The contemporary 15,000-ton Majestic class supplied a concrete benchmark for the size and power of the ships Japan expected to face.
Chapter · 38:27-42:28
Threat analysis behind the six-battleship standard
Japanese planners estimated the expeditionary force a major power could send to East Asia and set a fleet large enough to make that commitment painful.