How British intervention could have changed the Civil War

If the Union had invaded Canada during the American Civil War, how might Britain have responded?

Britain would reinforce Canada and likely use amphibious raids; breaking the Union blockade could restore Confederate trade and supplies, changing the war's balance.

A maritime response is more likely than a large British army marching directly on New York or Washington. Britain would reinforce Canada and could land amphibious raiding forces at several points. Early in the war, an attack on British territory could also have overcome London's reluctance to support either American belligerent. Royal Navy action against the Union blockade could then reconnect the Confederacy with overseas trade and supplies before Union industry and population became fully decisive.