Basically US involvement in the Napoleonic War in alliance with
France would end up with a similar situation to the War of 1812,
especially when Napoleon invades Spain.
With the Royal Navy being Superior to the French and American Navies
combined in the period of the Napoleonic war there would be little to no
chance of French support reaching the Americans across the Atlantic and
America would be too involved in fighting the British forces in North
America to send that much aid to France...not to mention the fact that
not every State in the Union wanted to fight Britain in the first place
in the War of 1812 when it was a personal quarell between the two
countries so I can't see them wanting to fight Britain just to help
Napoleon.
With the Royal Navy able to depend on Horatio Nelson until Trafalgar (as
that would probably happen regardless of the POD), Cuthbert
Collingwood, Thomas Cochrane and William Sidney Smith (just to highlight
some of calibre of the British naval commanders) it is unlikely that
the tiny US Navy would be able to do anything against the British
because in the War of 1812 they found it hard to even get out to sea so I
find it hard to believe that they would be able to cross the Atlantic
themselves.
In summary American involvement in the Napoleonic wars would just lead
to two different fronts being created for the British, the Peninsular
War and an American War, but I do not think that Britain would be unable
to deal with that.
