What if the French and Americans went to war with Britain in the 1840-1850's? It's not completely unlikely, both had issues with Britain at the time.
A two big things that would affect the USA, The Mexican American War (whether it hasn't started yet, being fought, or was just fought.
Many things could instigate the war. Some issues that could split the USA and Britain:
-An annexation of Texas by Britain (although unlikely)
-An escalation of a Aristook type conflict over the Maine border dispute.
-Who gets Oregon, or at least Victoria Island
-Earlier secession in Southern States and the threat of the British breaking the blockade.
-Fishing off the coast of Newfoundland?
-This forum
https://www.alternatehistory.com/discussion/showthread.php?p=2143879&posted=1#post2143879 suggested that there was a treaty that obligated Britain, France, and the USA to observe Hawaii's nuetrality/independance. Maybe that could escalate?
-Revenge for 1812. the USA is now much stronger, but so is Britain.
-Any combination of those issues.
With France, it would be amusing to have Napolean III go after Britain. The only issue is the whole Mexican puppet government he wants, that the USA could dispute. Depends on the year. Plenty of things could have sparked a war between Britain and Fance in this time period. What kind of effects would it have on the later Franco-Prussian war? Could be some interesting WWI butterflies...
Britain has little to gain, much to resolve...