The Battle of San Diego refers to the fourteen-week military campaign in and around the American city of San Diego during the 1925 Japanese invasion of the United States, beginning with the sustained bombardment of the city's naval facilities and harbor in early March up until the withdrawal of American soldiers east and south following the landing of a large Japanese continengent at present-day Imperial Beach. The battle was noted for its ferocity, and for the initial decisive success of the American soldiers at repelling a two-pronged Japanese attack, defeating the same large Japanese 10th Imperial Army that had just swept through the San Bernardino and Temecula valleys attack from the north while defending against a landing party of as many as 250,000 Japanese soldiers landing along the beaches as part of Long Land from as far north as Newport Beach and as far south as Rosarito Beach. The battle resulted in 150,000 American casualties (approx. 37,000 dead and 110,000 wounded) and almost a quarter of a million Japanese casualties (est. 80,000 dead and 160,000 wounded).
The battle devastated the city of San Diego and its environs, including the towns of Tijuana and Escondido. However, it proved critical to preventing the Japanese from establishing a more firm beachhead, and recouped the swift losses suffered by the Americans across Los Angeles and in its surrounding valleys. It also helped inflict terrible attrition on the Japanese, weakening Japanese morale and greatly diminishing their hopes to swiftly force an American capitulation with rapid movement eastwards from coastal California. The battle was supplemented by smaller skirmishes at Ensenada, where two Japanese landing attempts were rebuffed, and is regarded as one of the greater strategic victories in American history, although it was not a tactical victory in that the Americans were forced to retreat upon realization that nearly two million Japanese soldiers were headed for America's coast, a number against which the American defenses could not survive a battle against.