Key point: A nation's ability to wage war depends on its economy.
In January 1936 Japan announced its intention to withdraw from the London Naval Treaty, accusing both the United States and the United Kingdom of negotiating in bad faith. The Japanese sought formal equality in naval construction limits, something that the Western powers would not give. In the wake of this withdrawal, Japanese battleship architects threw themselves into the design of new vessels. The first class to emerge were the 18.1-inch-gun-carrying Yamatos, the largest battleships ever constructed. However, the Yamatos were by no means the end of Japanese ambitions. The Imperial Japanese Navy (IJN) planned to build another, larger class of super battleships, and had vague plans for even larger ships to succeed that class. War interceded, but had Japan carried out its plans it might have deployed monster battleships nearly as large as supercarriers into the Pacific.
The A-150 class would have superseded the Yamatos, building on experience with that class to produce a more formidable, flexible fighting unit. Along with the Yamatos, these ships were expected to provide the IJN with an unbeatable battle line to protect its Pacific possessions, along with newly acquired territories in Southeast Asia and China.
The A-150s would theoretically have carried six 510-millimeter (twenty-inch) guns in three twin turrets, although if problems had developed in the construction of that gun they could have carried the same main armament as the Yamatos. The 510-millimeter guns would have wreaked havoc on any existing (or planned) American or British battleships, but would also have caused substantial blast issues for the more delicate parts of the ship. The A-150s would have carried heavier armor than their smaller cousins, more than sufficient to protect from the heaviest weapons in the American or British arsenals. The secondary armament would have included a substantial number of 3.9-inch dual-purpose guns, a relatively small caliber suggesting that the A-150s may have relied on support vessels to protect them from enemy cruisers and destroyers.