War of 1812?
The war of 1812 was a response to the US invasion of Upper Canada and the burning of York (then the Capital city of the nascent Canada).
Or as I prefer to call it... The War of Southern Aggression ;)
The US formally declared war on Britain in response to trade restrictions.
Britain however had been aggitating for war by stirring up Indians in the Northwest to impede settlements of American citizens there and were impressing American citizens to service in the British Navy.
The U.S. felt all these were violations of international law and war was declared.