Before WWII, the US had a series of plans to fight potential enemies, including the unlikely event of war with the British Empire.  This plan to take on the Brits was called War Plan Red, and a subset of this plan was  the invasion of Canada, called War Plan Crimson.  Revelation of these plans upon their declassification caused some embarassment, but it also came out that Canada had also conceived their own plan to defeat the US (see "Defence Scheme No 1" for details).    


Dominion of Canada, in 1930's (note that the Canadian conquest of Newfoundland and Labrador had not yet taken place and these areas were still a British possession)




Here are some excerpts from the 1935 version of the plan, outlining some of the underlying strategic thinking (note- reformatted in places for easier reading):


"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...


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.

f. The reasons why these various areas are strategically important may be briefly summarized as follows:


(1) Halifax Monkton St. John Area. (Maritime Province) The port of Halifax is the key point in the area, for while the port of St. John affords excellent facilities for an overseas expedition, it is so close to the United States border that uninterrupted use by Red cannot be expected. At Monkton, the peninsula connecting Nova Scotia and the mainland narrows to 14 miles. With Halifax in possession of Crimson, this area affords the best defensive position to prevent any advance westward by Red.

(a). Control of Halifax by Blue would:

1. Deny Red the only ice free port on the east coast and the only ports, other than the St. Lawrence River ports, suitable as an overseas base.
2. Deny Red a prepared naval base on the east coast, from which to operate against Blue naval forces or commercial shipping.
3. Disrupt transoceanic submarine cable service between Crimson and Red (except from Newfoundland) and between Crimson and the West Indies.
4. Deny Red the use of certain air bases from which to operate against northeastern United States.

(b) The control of Halifax by Blue, renders the Port of St. John and the Monkton area of secondary importance. Failing to secure Halifax control of the Monkton area by Blue would:

1. Deny Red the use of St. John Harbor.
2. Cut the lines of communication between the port of Halifax and St. John and the remainder of Canada.
3. Place Blue directly across the only line of advance (by Red) from Halifax, on the shortest possible defensive line.
4. Deny Red the use of certain air bases from which to operate against northeastern United States.
5. Give Blue the use of various small air fields at Monkton and St. John.


(2) Montreal - Quebec Area (St. Lawrence River Area). The ports of Montreal and Quebec, while ice bound about four months of the year, still afford the best overseas base both as to facilities and location. In addition the area is of great commercial importance in that it controls all lines of communication, by land, sea and wire between industrial and agricultural centers of Canada and the eastern seaboard. While Montreal has the larger and more commodius harbor and terminal facilities, Quebec, due to its physical location, is the key point of the area. Control of this area by Blue would:

(a) Deny the use of all good St. Lawrence River ports to Red.
(b) Cut all Canada, west of Quebec, viz. industrial, and agricultural centers from the eastern seaboard.
(c) Deny Red and Crimson and make available to Blue, the principal air bases in eastern Canada.
(d) Deny Crimson coal and iron from Nova Scotia and Newfoundland as well as all imports via the Atlantic.


(3) The Great Lakes Area. This area comprises several critical points:

(a) Niagara River crossings and Welland Canal.
(b) The waters connecting Lake Huron and Lake Erie.
(c) The great industrial area of Canada - that part of Ontario lying between Lake Huron and Lakes Erie and Ontario.
(d) The waters connecting Lake Superior and Lake Huron, including the Soo Locks.
(e) The Sudbury nickel-copper mines. Control of the Great Lakes waterway is vital to Blue, for the transportation of iron ore, coal and grain and such control will necessitate occupation of a bridgehead covering the narrow boundary waters at and near the Soo Locks and in the Detroit Area. The bridges over the Niagara River and the Welland Canal, connecting Lake Erie and Lake Ontario are of importance to Blue for occupation of the Important industrial area of the Niagara-Ontario peninsula. The Welland Canal would become of importance as a line of communication if Blue seized the peninsula. While control of that area is of importance in crippling Crimson industry, it is probably of greater importance in denying the enemy Crimson and Red, a most convenient base for operations against highly industrialized areas in the United States.


(4) Winnipeg Area. Winnipeg is the nerve center of the transcontinental railroad system. Control by Blue will effectively separate eastern and western Canada and block transportation on men, grain, coal, meat and oil to the east. The completion of the Canadian National Railroad to Churchill Manitoba on Hudson Bay and the development of the port at Churchill provide an alternate route to Europe via Moosonee, Ont., and the Tem. and Ont. Ry. to northeast Ontario. While the water route through Hudson Bay is only open about four months of the year, and the ports are supplied by single track railroads, a considerable amount of traffic could be developed in an emergency.


(5) Vancouver - Victoria Area. As pointed out above, the ports in this area are of secondary importance only under the conditions, which may reasonable be assumed. However, the area has certain military importance, due to the naval base at Esquimalt, and is a possible outlet for the Canadian plan provinces and western Canada. Its control by Blue would deny the enemy any base or outlet on the West Coast; simplify the problem of protecting our shipping in the Puget Sound area; and interrupt cable communication with the far east. While Prince Rupert, B.C. has an excellent harbor and terminal facilities with good rail connections leading east, naval blockade of this port would be readily possible, once the Vancouver - Victoria area was in Blue control."

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


Additional resources:

Opens pdf of War Plan Red, from May 1930


Raiding the Icebox, Washington Post article

What's that line about being friendly towards everyone but have a plan to kill them?

The Naval War College did a lot of wargaming and planning. From it they learned that it was impossible to reinforce the Philippines for two years.  By the same token, Orange (Japan) could not invade the Continental United States. The logistics were too staggering and it would take that much time to build a support fleet for the fighting fleet.

IIRC, that was War Plan Rainbow.

Found it facinating that the Pacific War followed it damn near exactly.

It's my understanding that we've got plans to invade damn near everywhere.

Useful when need presents, and useful as an exercise even if it never does.

Whether invasion of Canada or defense of Canada, the targets are the same.

For the most part, not much has changed.  Canada is unable to defend any border other than perhaps the southern border.  The real defense lies in the geography and logistics.   Nothing really has changed since then.

Insomuch as the Great Lakes, the targets are largely the same as 100 years earlier in the war of 1812!