War Plan Red (properly the Joint Army and Navy Basic War Plan - Red) was the American plan for a war with the British Empire. It was developed in 1930 by the Joint Army and Navy Board, the main organisation which developed war plans for the US military. It was part of the series of 'Rainbow' war plans, the US's colour-coded plans for wars against pretty much the entire world. The most famous of these plans was War Plan Orange, the plan for a war with Japan; others included War Plan Black, for Germany, War Plan Green for Mexico and War Plan Yellow, for operations in China.

Plan Red was, fundamentally, a defensive plan. It envisaged the United States being attacked by Britain, likely involving an invasion of America from Canada. This sort of 'out of the blue' attack was part of the reason why it was produced; the US needed some sort of idea of how to respond to a rapid, or even a surprise, attack. Beyond that, it was part of a long tradition of the US making war plans for a possible war with Britain. Following the visit of the Great White Fleet to Australia and New Zealand in 1908, the USN used the experience to produce plans for an invasion of Australia. Towards the end of WWI, the US Navy's planning section was considering Britain to be a likely future adversary. However, the main impetus behind Plan Red was the US Army. The US Navy's planning in the 1920s was dominated by Plan Orange, for a war with Japan. This would almost inevitably be a naval campaign, with little call for the US Army. The Army, wishing to retain experience in developing war plans and justify its own procurement, advocated for planning for a war with Japan and Britain together (Britain and Japan were allied until 1923, making this less unlikely than it sounds). War Plan Red would grow out of these plans in the 1920s.

Plan Red assumed that Canada would be a belligerent on the British side. Should Canada declare neutrality, the plan called for this to be rejected unless the Canadian government gave significant material guarantees, the most significant of which was American occupation of the main Atlantic ports. This made sense given that the planners assumed that the war would start with Britain invading the USA to destroy or capture vital war industries, in the hope of preventing the US from bringing its greater industrial potential to bear. Canada was a clear and obvious base for such an attack. As a result, the first objective of Plan Red was the capture or neutralisation of Canadian ports, especially Halifax; this would prevent Britain from bringing reinforcements into the theatre. Following this, various strategically important areas of Canada would be occupied. Montreal, Quebec and the waterways of the Great Lakes were the most important targets, as their capture would all but eliminate Canadian weapons production and protect US trade on the Lakes. Winnipeg was the next target, splitting Canada into two parts which could be occupied at will. With Canada captured, the hope was that this would force Britain to seek peace; there were no plans for a strike against Britain.

The US Navy's role in the plan was limited. The idea of using the USN to strike against British trade was examined and quickly discarded. While it would be disruptive to Britain's economy, it risked offending neutrals, and would allow Britain to reinforce Canada, making the Army's role harder. Trade warfare was also seen as ineffective, thanks to the influence of Alfred Thayer Mahan, the most important American naval theorist of the 19th Century. The idea of trying to defeat Britain in a Mahanian 'decisive battle' was also rejected. While the USN and Royal Navy had similar strength, after the Washington Naval Treaty, Britain's fleet was seen as being better balanced for a fleet action. The RN had more fighting experience, and its four fast battlecruisers provided a significant tactical advantage in fleet actions, able to use their speed to flank the American line. A decisive victory in a fleet action would, for Britain, greatly offset the loss of Canada, and hand Britain the strategic initiative. As a result, the USN's role in War Plan Red was to form a 'fleet in being'. It was to be concentrated in the north-west Atlantic, and whittle down the RN with submarines, aircraft and small raids; once favourable conditions for a fleet action were present, one would be fought on the USN's terms. With a victory secured, the USN could then target British trade and colonies in the Western Hemisphere.

There were two British equivalents to Plan Red; a loosely formulated Admiralty plan, and Canada's Defence Scheme No. 1. Defence Scheme No. 1, mainly produced by Colonel J. Sutherland 'Buster' Brown, head of the Canadian Army's intelligence section in the 1920s, was the sort of aggressive plan the US expected to be facing. With it clear that Canada had no hope in a prolonged war, nor on the defensive against the larger US Army, the scheme called for Canada to mobilise before the US could. Flying columns would cross the border as soon as war was declared, followed a few days later by larger formations. Initial targets included Spokane, Seattle and Portland in the west, Fargo, Minneapolis and Saint Paul in the centre and Albany and Portland (Maine) in the east. The hope was that Britain would then be pouring reinforcements into Canada during the time brought by the capture of these locations. The Admiralty's plan, meanwhile, assumed that Canada was a lost cause. Canada could not be held without reinforcements, and sending them would only lead to a land campaign where American held the advantage. As such, the Admiralty's plan was more interested in protecting Britain's trade and defeating the American fleet; British reinforcements would only be sent to Canada to hold Halifax as a base for the fleet to operate from. Bermuda would be the likely main base for the British fleet, while Halifax, Jamaica and St Lucia would be used as bases for cruisers and submarines to attack American trade. The hope was to force the USN to fight a decisive action, this time on Britain's terms. Forces in the Far East would be used to secure Manila in the Philippines, preventing the USN using it to attack trade or British colonies there - this would use available RN forces plus manpower drawn from India and Australia. The Admiralty's main worry was that the USN would cross the Atlantic, using a base in a European ally or seizing one from a minor power like Spain or Portugal. This would put the RN in a difficult position; from such a base, the USN could easily cut the shipping routes Britain depended upon, while the RN could not easily destroy the American fleet without risking a disastrous loss. While the Canadian Defence Scheme played almost exactly into the hands of War Plan Red, the Admiralty plan would have been very hard for the USN to counter. Meanwhile, the USN was not planning to do what British planners feared most.

While these plans were produced, risk of war was low. The British mostly expected war with the US to accompany a war in Europe. War would be provoked by Britain's attempts to blockade its enemies in this war. The US, meanwhile had some worries that Britain would attack in the hope of crippling its main economic competitor. However, war with Japan, which was taking a more aggressive stance against American interests in Asia, was seen as more likely.