As a Washingtonian i'm insulted by the idea we'd side with the axis.
Don't worry, they won't do it for long once the rest of the US is finished curb stomping them. Assuming the federal government would even let them secede in the first place, which history indicates is doubtful.
Most Cascadian nationalists seem to work under the assumption that the federal government will eventually collapse, but the pacific northwest will be slightly better off because we don't make the mistakes the same mistakes as everyone else. I've heard some things about the region being somehow less vulnerable to climate change for various reasons, and I've also made jokes about how we might end up having to build a wall across Oregon to defend ourselves from thirsty Californian refugees. However in the event of a nuclear war we'd be fucked because there are so many military bases out here.
Also most Cascadia plans involve Washington, Oregon, and Vancouver. Sometimes even parts of Idaho and California. The state of Washington alone does not make a very good country, even with Germany somehow helping from the other side of the world.
Now i almost want to make my own Cascadia AU. A better diverging point would be the Union allowing the south to succeed, resulting in a less hegemonized north america. Without the civil war, the union isn't as militarized, so they aren't able to project force westward. As a result the Indian tribes are able to meet the settlers on their own terms, and end up modernizing instead of being wiped out. The united states would be divided into north, south, mormonland, some combination of texas/california/mexico, The kingdom of Hawaii, and a federation of native tribes in the pacific northwest.
In the next century things get interesting. Assume some other power modernizes japan (I think that's a fair assumption) and you get Russia and Japan as the local superpowers. Do Cascadia/California/Hawaii form an alliance for mutual protection, or do they get subjugated by imperial japan? Does Russia kick everyones ass without the US to stop them?