[Sidney Crosby's Hat]

The U.S. drafted up an invasion plan in 1935:

http://www.glasnost.de/hist/usa/1935invasion.html

In case of war with the United States, Canadas coal imports from this country would be cut off and her railroads and industrial activities seriously handicapped. If Blue controlled the Quebec area and Winnipeg, Canada's railroads and industries dependent upon "steam power" would be crippled.

Nickel is necessary to industry and indispensable in war. Control of the Sudbury mines, in case of war, is therefor of vital importance.

Also - sounds like Alberta is of little concern!

Crimson cannot successfully defend her territory against the United States (Blue). She will probably concentrate on the defense of Halifax and the Montreal-Quebec line in order to hold bases of operation for Red.

Important secondary efforts will be made to defend her industrial area and critical points on her transcontinental railroad lines.

8. Areas of Strategic Importance.
Analysis of the above data and discussion indicates certain areas which
would become of considerable military importance in the event of war with
Red; namely,
a. The Halifax Monkton St. John area, sometimes called the Martime
_
Province area.
b. The Montreal Quebec area, sometimes called the St. Lawrence Area.
_

c. The Great Lakes Area.
_
(1) Niagara River Area.
(2) Sarnia-Windsor Area.
(3) Sault Ste. Marie Area.
(4) Sudbury Area.
d. Winnipeg Area.
_
(1) Winnipeg City and vicinity.
(2) Churchill, Manitoba Area.
e. Vancouver-Victoria Area.
_
(1) Ports of Vancouver and Victoria, area.
(2) Prince Rupert area.

EDIT: And it was pretty seriously close to happening: http://mostlywater.org/u_s_war_plans...sion_of_canada

In March 1935, General Douglas MacArthur proposed an amendment making Vancouver a priority target comparable to Halifax and Montreal. This was approved in May 1935, and in October 1935, his son Douglas MacArthur Jr. began his espionage career as vice-consul in Vancouver. In August 1935, the U.S.A. held its then largest ever peace time military maneuvers, with more than 50,000 troops practicing a motorized invasion of Canada, duly reported in the New York Times by its star military reporter, Hanson Baldwin.