The Imperial Japanese Navy (IJN) is a recurring antagonist organization from the 1940 series, based on the real navy which existed up until the end of World War II.
The Imperial Japanese Navy features prominently during the first two titles in the series, 1942 and 1943: The Battle of Midway, where they act as the antagonists. In both games, the player controls the Super Ace, a U.S. Navy pilot whose mission is to fight through the IJN's forces. Most enemy ships in these games carry the IJN's ensign on their wings.
Due to the controversy regarding the depiction of the Japanese as enemies (since the series was created by a Japanese company for a Japanese audience), later games in the series drop the IJN in favor of using Nazi Germany as the main enemy force, though many of IJN's fighters still appear on them.