The Oregon Theater (not to be confused with the Alaskan War's Columbia Theater) refers to the theater of operations in the Pacific War generally referring to American defensive maneuvers against the invading Japanese in the summer and early fall of 1925 in the state of Oregon and the southern Washington state. Some include northern California as part of the Oregon Theater (especially Japanese military historians) as many attacks on northern California were initiated by divisions that landed as part of the Japanese landings in Oregon, and some military historians also regard the attempted Japanese beachheads on Vancouver Island and the northern Olympic Peninsula to be unfulfilled operations of the Oregon Theather.
In general, military historians regard the Oregon Theater to have been a poorly executed component of the Japanese invasion of the United States, as it withdrew men and materiel from the more successful operations in Southern California, where the Japanese enjoyed both tactical and stragetic success deep into September. The difficulties of the Japanese in crossing the Coastal Range in Oregon as well as their struggles in penetrating deep into Washington bought the American 8th Army time to set up defenses both in and around Tacoma as well as in central Oregon. While the Japanese were able to secure most of central Oregon and the critical city of Vancouver, they were unable to replicate this success in Washington and were forced to retreat. Disastrous losses at Monticello and throughout Oregon forced the Japanese into their disastrous retreat across the Coast Mountains in October of 1925, and was followed shortly thereafter by American victories throughout California in October, November and early December.