Britain’s response depends on why Canada was annexed

What would Britain do if an otherwise allied United States annexed Canada?

The cause would determine Britain’s response, but an unprovoked American attack on Canada would demand a strong reaction rather than quiet acceptance.

Britain’s response would depend first on why the annexation happened. A war of aggression, a Canadian political crisis, and a Canadian attack on the United States would create different British choices. The pre-existing alliance cannot determine the response once the annexation itself has transformed the political relationship. For an unprovoked American attack, Britain would immediately challenge the action rather than treat it as normal allied behaviour.