Alaska
The Californians immediately attacked the British but the Royal Navy made landing near impossible so the Californians left Sitka to the British. The Californian troops (with some settlers) invaded the Northwest territory. It was then the British declared war on California and the alliances came into play. California was then invaded by the Americans while Russia landed troops in Alaska to aid in the Invasion of Canada. The Californians held the Americans in the Rockies and even diverted some troops North to aid in the quickly advancing invasion force.
Russia is invading Canada... through Alaska.
I don't even have words for how impossible that is...