Japanese Alaska..
Yes, many moons ago on this Forum I wrote a timeline in which Russia ceded Alaska to Japan as part of the Treaty of Portsmouth.
Alaska (or its Japanese name) was nominally ruled by a Governor and held by one of the younger sons of the Emperor. In the 1920s and 1930s it was settled by colonists (often forcibly) and was used extensively by the Imperial military as part of its preparations for war.
As the attack on Pearl Harbor occurred in December 1941, it was impossible given severe winter conditions for any serious activity to occur on the Alaskan-Canada frontier.
In the spring of 1942, however, Japanese naval units based on the Alaskan coast launched a series of raids on Canadian and American coastal communities including Vancouver and, most daringly, the Great Bay Raid of April 9-12 1942 in which a small Japanese force landed north of San Francisco. Although not a serious military incursion, the attack was believed to be an invasion and led to panic in much of California for forty-eight hours until reserve units were able to hunt down the raiders and kill them.
As the Americans regained control of the Pacific from 1942-44. the Japanese in both Alaska and the Aleutians became increasingly isolated and by early 1945 American forces had captured most of the Aleutians leaving the Japanese in Alaska isolated. An American raid in June 1945 led to a rapid retreat away from the main towns. The Japanese Army in Alaska capitulated on August 15th 1945.
The Russians made a postwar claim on Alaska but, following Russian intransigence in Europe, the Americans rapidly moved to establish control up to the Bering Strait. Even today, Moscow retains a claim on Alaska but the area is now generally recognised as part of the United States.
(I had an alternative version in which Russia regained Alaska in 1945 and set up the Autonomous Republic of Alaska. The Canadian/Alaskan frontier became as heavily patrolled and fortified as the divisions of both Germany and Korea but although roads were improved, the main access continued to be by air and sea.
In 1991, Moscow began to withdraw its forces from Alaska and allowed more investment and tourism. This encouraged the Alaskans to seek more autonomy from Moscow but since the election of President Putin, central control has been re-established though tourism and external investment continues to be attracted into the region).