The Imperial Japanese Navy (Nihon Kaigun) in 1941 was unquestionably a world power. With the help of the British, who trained the Japanese to the point that all bridge orders were issued in English, a Japanese fleet was built in the early twentieth century. At first the Japanese Imperial Navy was built overseas in England, but soon Japanese yards were building Japanese ships based on English designs. Soon Japanese ships were native designs equal or better to anything afloat.
The goal was to emulate the nations the Japanese perceived as the most powerful nations. The Navy emulated the reigning sea power, England, while the Army modeled itself on the Imperial German Army. When the two European powers went to war in 1914, Japan joined the Allies but expected Germany to win. The Army was surprised by Germany's defeat, but the nation was happy to occupy Germany's Chinese and Pacific possessions.
The Treaty of Versailles and the later Treaties of Washington (1920) and London (1930) limited the Japanese. The Navy was upset with the officers who negotiated the treaty, including Isoroku Yamamoto. The officers countered with the charge that a war with the west would be suicidal, because of the superior industrial output of the west.
Yamamoto was a vocal advocate for peace, and was targeted for assassination by the right wing extremists. Nagano, the Supreme Commander, transferred him to take command of the Imperial Japanese Navy, which saved his life.