Co-Prosperity Sphere:
The Imperial Japanese Navy
by Mike Bennighof, Ph.D.
July 2020

Our Great Pacific War: Co-Prosperity Sphere expansion book is drawn from our Long War setting, originally an alternative-history story arc for the Second World War at Sea game series. In the Long War, Germany attacks the Soviet Union in 1940 and only turns on the Western democracies after the Soviets are defeated. War does not come to the Pacific theater until 1943.

When it does, it is a far more powerful Japanese Empire that meets American aggression. Japanese provinces include Manchuria, Taiwan, Korea, Shantung, the Philippines and Hawaii; Japanese colonies range from Western Samoa to Hainan. Japan boasts a greater population than the United States, though she cannot match American industrial and financial power, but she has tied the Chinese Empire tightly into her political and economic orbit.

You can read more about the background here.

Japan’s great economic strength, combined with plentiful cheap petroleum, has resulted in a much larger Imperial Japanese Navy than that of our own reality. That’s going to be the basis for a massive expansion set for our naval games, but also gives the Japanese a great deal more striking power in Co-Prosperity Sphere.

Along with the Empire, the Imperial Japanese Navy expanded in the years after the Russo-Japanese War, building additional dreadnoughts and battle cruisers. After defeating China in 1915, the Japanese incorporated the most effective Chinese warships into their own fleet, including dreadnoughts built in British, German, American and Austro-Hungarian shipyards. During the Russian Civil War the Japanese seized the Tsar’s Pacific Fleet, taking over the handful of ships worth bringing into service.

Note: The “War of the Dragon” with China was the subject of our limited edition Great War at Sea: Rise of the Dragon supplement, returning soon in a new edition.

The enlarged Empire commanded greater resources, and continued to pour them into its Navy. The Fuso, Ise, Nagato and Tosa classes, initially planned to number two ships each, instead became sets of four battleships.

Battleship Tosa, just after launch.

Thanks to her enhanced power and prestige, Japan was able to demand and receive equal treatment at the Washington Naval Conference, extracting an equal ratio with Britain and the United States. That allowed the Imperial Japanese Navy to complete and retain the new Tosa-class fast battleships plus all twelve units of the Fuso, Ise and Nagato classes; they also obtained a provision allowing conversion of all four new Amagi-class battle cruisers into aircraft carriers and six smaller, older battle cruisers into light carriers. A small aircraft carrier already under construction was cancelled.

Two of the smaller ships were initially converted, with the others laid up to await test results from the first pair. As experience taught Japan’s naval architects the intricacies of carrier air operations, the remaining ships were rebuilt until all 10 were complete in the early 1930’s.

During the decades between the wars, Japan built a naval air arm to match its carrier fleet, with a large reserve of well-trained pilots (thanks to cheap and plentiful gasoline) and cutting-edge, high-performance aircraft. The converted battle cruisers would be joined by the Soryu class of three large purpose-built carriers laid down in 1934, three more of the Shokaku class laid down in 1938, and finally three of the Taiho class begin in 1941.

Japan began to build battleships again after 1936, laying down four huge ships of the Yamato class with 18-inch guns. Trusting a great deal in their carrier air arm, the Navy also built more heavy cruisers to accompany the carriers and a very large class of small anti-aircraft cruisers to help protect the flattops.

American politicians heated up their “America First” rhetoric following the 1940 election of Indiana Governor Paul McNutt – who advocated “the extermination of the Japanese in toto” – as president. In response, the Imperial Navy activated its “Rapid Armaments Program,” laying down additional aircraft carriers of the new Unryu class plus escorts, destroyers, transports, submarines and mine warfare vessels. Long-standing plans to convert a number of passenger liners, seaplane carriers and submarine tenders into escort aircraft carriers also went forward.

Notes: McNutt unsuccessfully challenged Franklin D. Roosevelt at the 1936 and 1940 Democratic National Conventions. He really did demand a genocidal campaign against Japan during a 1945 speech, when serving as chairman of the War Manpower Commission. Offered a chance to “clarify” his remarks, he repeated that all Japanese, civilians included, should be killed.

Japan did launch such an emergency naval expansion program in August 1941, but this bigger and richer Empire is able to launch a bigger program.

By late 1943, the Imperial Japanese Navy had ten fleet carriers and fourteen light carriers in commission, with three more fleet carriers nearing completion and six more under construction. A single unique “battleship-carrier” also remains in service, though relegated to support missions. Modern aircraft as capable as anything in the American arsenal filled their flight decks: A7M “Reppu” fighters, D4Y “Suisei” dive bombers and B6N “Tenzan” torpedo bombers, with the new B7A “Ryusei” attack plane beginning to replace the latter two types on the bigger Japanese carriers.

Notes: The initial impetus for our Long War setting didn’t actually come from our Plan Z expansion set; we had published Leyte Gulf with all of these modern Japanese aircraft that never saw use, or did so with green pilots and in tiny numbers compared to the huge American armada. I wanted to get them into actual play; I also wanted to find an excuse to re-use the Agincourt battleship-carrier drawing from our Golden Journal.

Supporting the carriers are four battle cruisers rebuilt as fast battleships, and a battle line of sixteen reconditioned battleships from the Great War era plus the four behemoths of the Yamato class. Two older battleships, German-built prizes taken from the Chinese in 1915, are in service as training ships and available for secondary duties.

From its huge, well-protected base in the Pearl River Estuary on Oahu in the Hawaiian Islands, the Combined Fleet commands a dominating central position in the Pacific basin. The Americans are known to covet the islands, but with their main fleet base at San Diego over 2,600 miles away, there does not appear to be much cause for alarm.

In game terms, the Japanese begin the 1943 scenario and campaign game in Co-Prosperity Sphere with a large surface fleet and a great many aircraft carriers, and additional landing craft (vital to power projection in Great Pacific War). The Americans get to start with most of those surface and aircraft carrier pieces on the board (in the West Coast box) instead of in the future-years force pools as in the historical scenarios of Great Pacific War.

The dice are going to roll from the first moments of Co-Prosperity Sphere. And that’s its reason to exist.

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Mike Bennighof is president of Avalanche Press and holds a doctorate in history from Emory University. A Fulbright Scholar and NASA Journalist in Space finalist, he has published over 100 books, games and articles on historical subjects. He lives in Birmingham, Alabama with his wife, three children and his dog, Leopold. Leopold is a good dog.