Crimean war is inconclusive, but Russia comes out embittered against GB.
GB goes to war against Union in ACW over the Alabama incident and then Russia comes in on Union side. ACW become worldwide conflict and lasts until 1900 with Union/Russian victory
How plausible?
A 40 year war? Um, sorry, old boy, but no.
I remember that the Russians made a favourable impression by dispatching their Pacific Fleet to visit the Californians during the ACW, because they wanted to be sure that if there was a war, they wouldn't be iced in. So that could make things interesting in the Pacific for a year or so. The USN on the East coast is boned. The best they can hope for is to defend a few major harbours with primitive Monitors. Their trade is gone; their supply of British new and surplus munitions and materiel is gone; their cash is gone.
So they have no powder for their guns. This is the most important thing. We've looked at this in previous
Trent Affair threads, and the Americans only have a year's worth* of powder. Before anyone points out that the Americans can make their own - yes, they can. But not enough to supply the fighting needs of armies operating in and around Canada and the Rebel States for
years.
Generously, I can see the Russians wreaking a fair bit of havoc for a year or two until the RN floods the Pacific with ships. Generously, I can see the Union achieving a stunning victory against the Rebels in '62 - but this won't break them. Grant could take Vicksburg, and that would be a bugger for them, though. However, troops from his army and from the AoP will be sent to Canada. So what's needed is to get the Union with fewer supplies and no cash to achieve greater victories than IOTL against more, better supplied and better trained enemies attacking them on multiple fronts.
Now this
could happen. I shan't rule it out. I think it very unlikely, but it
could happen. But there's
no way to keep that going for four decades. America would be in ruins, Russia would probably collapse - note the emancipation of the serfs is about now, and that required a lot of internal energy.
A brief disastrous war is best for the USA, if you want a drawn out conflict. Then we have a build up of a large professional army and navy for security. None of this revenge stuff we see in Turtledove's books, mind; rather an awareness of the fragility of her position. Then you can have another crisis (somewhere down the line) trigger another war and America will have the men and ships to fight it, and she'll have undertaken to acquire either stockpiles of powder and other necessaries or worked out how to make her own - the Rebels used bat guano from caves IOTL. I'm still not sure about Russia, since she's internally distracted . . .
Anyway, no to forty years of war. But a qualified yes to a conflict described by later historians as The Forty Years War, which is actually a series of briefer wars between the groups.
* It might be two, my memory's a bit fuzzy, and I forget whether the calculation was for Union armies IOTL or included the formation of new units to fight in Canada.