https://en.wikipedia.org/wiki/War_Plan_Red

A key move was a joint US Army-Navy attack to capture the port city of Halifax, cutting off the Canadians from their British allies. Their next objective was to “seize Canadian Power Plants near Niagara Falls.” This was to be followed by a full-scale invasion on three fronts: from Vermont to take Montreal and Quebec, from North Dakota to take over the railhead at Winnipeg, and from the Midwest to capture the strategic nickel mines of Ontario. In parallel, the US Navy was to seize the Great Lakes and blockade Canada’s Atlantic and Pacific ports.

The first strike with poison gas against the port city of Halifax was used to seize it, preventing the Royal Navy from using the naval base there, and cutting the undersea cable through Halifax, severing the connection between Britain and Canada.

American war planners had no thoughts of returning captured British territory: “The policy will be to prepare the provinces and territories of CRIMSON and RED to become U.S. states and territories of the BLUE union upon the declaration of peace.”