U.S. forces would take the port city of Halifax before seizing power plants near Niagara Falls. More American troops would storm across the border to capture Montreal, Quebec and other key cities. The secret U.S. military plan, known as War Plan Red, even authorized the use of poison gas.
The assault was never seriously considered. Drafted in 1927 and declassified decades later, it was a contingency plan for a highly unlikely war.
History took the opposite path: In the century since, the United States and Canada have been two of the friendliest and most integrated nations ever to share a border.
Until recently, that is.
President Trump has thrown the U.S.-Canada relationship into its greatest crisis since his country’s northern neighbor became self-governing in 1867. Accusing Canada of unfair trade practices, Mr. Trump has instigated a trade war that on Tuesday led Prime Minister Mark Carney of Canada to retaliate with tariffs targeting roughly $20 billion in U.S. imports. Mr. Trump in turn escalated with bans on imports of some Canadian motorcycles, alcoholic beverages, dairy products and molasses.
Other heated cross-border disputes have flared on matters ranging from Alaska’s Yukon border to whether Canada would allow American nuclear weapons on its soil.