It is the 1930's. The world is gripped by the Great Depression and militarism, as "great powers" in Asia and Europe turn to military expansion as a solution for their economic troubles. World War Two is not far away. The US sees potential military threats to itself against both coasts from Asia and Europe, and draws up contingency plans for a naval and land war against expansionist empires in both the Atlantic and the Pacific.
But the trans-Atlantic target of "War Plan Red" is not Nazi Germany or the Soviet Union. It is the British Empire.
Until 1917, the US was viewed as a third-rate nation with, admittedly, a growing economy, but no military power to speak of and no influence on world affairs--more or less as we view Indonesia or Brazil today. The US had managed to beat the aging and decrepit Spanish Empire in 1898, and had sent some troops to ineffectually chase Pancho Villa around Mexico in 1916, but it was, in the world of powerful nations, a nobody.
But during the First World War, the US had proven its economic might, and had, in the space of just two years, developed a navy and army that could hold its own against the most powerful nations on earth. By the 1920's, the US was firmly established on the world stage.
In accordance with its new role, America now drew up a number of military contingency plans against a series of potential opponents. These were give color-coded names, with the US itself as "Blue". Although there were many such plans, targeted against countries ranging from Iceland to China, only a handful of them were seriously developed. War Plan Black targeted Germany, and made plans for a re-match of the First World War (this was drawn up even before the Nazi Party came to power). War Plan Orange dealt with Japan, which was rapidly building a huge fleet and becoming a naval rival in the Pacific. War Plan Green was geared towards a war with Mexico, a politically-unstable neighbor which the US had already invaded in 1846 and had sent troops to intervene several times since. War Plan White was a contingency to use the military against domestic uprisings within the US--these plans at first focused on a potential Socialist or Communist revolution, and then in the 1930's added the possibility of a home-grown Nazi uprising.
The most surprising plan, though, at least today, was War Plan Red, which targeted the British Empire. Although the British Empire was the largest in the world, the US was already shouldering aside Great Britain as the world's economic and military superpower--and England, it was assumed, may not go quietly. Throughout its history, Britain had always responded to global challengers to its power by simply crushing them, and US planners prepared for a repeat of that policy. "The war aim of RED in a war with BLUE." the War Plan Red noted, "is conceived to be the definite elimination of BLUE as an important economic and commercial rival."
The US war planners assumed that any war between the US and UK would center around the British possession of Canada. The US Army prepared for a land war that would be launched by British forces from Canada (code named Crimson) to try to seize strategic American locations like Detroit and Buffalo and then move on to New York and Washington DC. The US Navy planned for a potential British amphibious assault across the Atlantic, with invasion landings in New Jersey and Delaware, and also for a US blockade of Canadian ports on both coasts, to prevent British troops and supplies from reaching Canada. The US aims in War Plan Red were to prevent an invasion of the US by invading Canada. Once conquered, Canada was to be incorporated into the US: "Blue intentions are to hold in perpetuity all CRIMSON and RED territory gained . . . The policy will be to prepare the provinces and territories of CRIMSON and RED to become states and territories of the BLUE union upon the declaration of peace."
The US thrust into Canada was to have three prongs. In the west, US forces would invade British Colombia and seize Vancouver, cutting off Canada from all of the British possessions in Asia. In the center, American troops from North Dakota would seize Winnipeg, and US Navy forces would seize all the Canadian territories around the Great Lakes, cutting Canada in two and controlling its industrial centers. And in the east, US forces from New York would capture the port of Halifax and the Maritime Provinces, cutting Canada off from Britain. All of these American attacks would be supported by massive planned air raids on Canadian cities using mustard gas. If the war with England still continued after Canada was successfully conquered, a US Navy thrust would then seize the British island possessions in the Caribbean, and trans-Pacific operations would take control over the British possessions in Southeast Asia.
In 1935, the US began building a series of army bases and airfields along the US-Canada border, put there to help carry out War Plan Red. Many of them are still there, including Fort Drum, in upstate New York. In the 1930's, Fort Drum was the site of large American war games which practiced implementation of War Plan Red. (Today, it is used by the US as a training area for Arctic and mountain warfare.) And in the early 1930's, famed aviator Charles Lindbergh was instructed by the US government to photograph Canadian beaches and airfields in Hudson Bay, as part of the intelligence-gathering for the potential invasion of Canada.
In the end, of course, the US ended up fighting Hitler and Mussolini instead of the British Empire, and England today is the US's staunchest ally (though the Canadians still think the US is a little nutty).
War Plan Red was declassified in 1974 (though parts of it, including three secret US airfields on the Canadian border, had accidentally been revealed in 1935). At about the same time, moreover, it was revealed that Canada too had its own contingency plans for war with the US, drawn up in 1921, code-named "Defense Scheme Number One". It called for Canadian forces to mount a "forward defense" by actively invading the US at the first signs of American aggression, targeting Albany, Minneapolis, Great Falls and Seattle, to occupy as much US territory as possible and hold out until British aid could arrive across the Atlantic.