They've done that twice, during the Revolutionary War and the War of 1812.
The US invaded Canada during the Revolution and 1812 wars too.
I think that the prevelence of Canadians in medical clinics in the northern tier states is a cunning plan to gather intelligence on the US.
Then again, it could be a mark of the failure of the Socialist Canadian pseudo-healthcare system.
No blood for syrup!
"They've done that twice, during the Revolutionary War and the War of 1812."
Actually three times, if you count the Confederate raid against St. Albans, Vermont which was launched from Canada.
It was alleged the Canadians were in cahoots (Great Britain being an unofficial ally and supporter of the Confederacy). Canada denied this, but refused to take any action against the raiders.