Answer:
At starting the war was between Britain and France in 1812 and America served as a neutral base for both countries fighting. The occurrence that made Britain to get into this War full-fledged was when America started standing against Britain and was favouring France for Britain's defeat.
This happened due to the Britain capturing the American ships and enforcing laws on trading with only Britain and British colonies. Imprinting laws over Americans on American lands led for the rebel Americans stood for it.