At of the turn of the 20th Century, America was coming into its own as a world power. In preparation for its new place in the world, the Joint Planning Committee, predecessor to the Joint Chiefs of Staff, drew up plans for war with likely opponents of the United States. Those plans went through numerous iterations and eventually became known as the Rainbow War plans because all the countries in question were assigned a specific color. The U.S. was then, as now, colored blue. What was started as a way to organize conops in the early 1900s became the basis for many of the decisions made during World War II.
The color plans ranged from small and unlikely to global and insightful. War plans such as Purple, Violet, and Grey covered American operations in Central and South America. War Plan Tan involved an intervention in Cuba. War Plan Brown was a plan for suppressing insurrection in the Philippines. There were also war plans Green and Yellow which dealt with Mexico and China respectively. They planned for domestic uprisings overseas that would have to be quelled by the U.S, like a repeat of the Boxer Rebellion in China.
America's World War I allies, Britain and France, were also covered. War Plan Gold was America's contingency for dealing with a belligerent France or her Caribbean colonies. The threat from French Caribbean colonies even led to the U.S. Army creating two special units, the 550th Airborne Infantry Battalion and the 551st Parachute Infantry Battalion. These were tasked with taking the island of Martinique in the event that the island became hostile. War Plan Red and its sub-plans Crimson (Canada), Scarlet (Australia), Ruby (India), and Garnet (New Zealand), were the American plans for war with the British Empire.
The Rainbow Plans had two components. First, it planned for the U.S. to have allies. Second, the U.S. would "prevent the violation of the letter or spirit of the Monroe Doctrine by protecting that territory of the Western Hemisphere." Based on these premises five Rainbow plans were developed. Plans 1, 3, and 4 were designed to protect the Western Hemisphere without the commitment of American forces to offensive operations and varied only in their scope of the projection of forces for the defense. Plan 2 called for supporting Great Britain and France against Germany with minimal American forces while the bulk of American forces carried out what was essentially War Plan Orange in the Pacific against the Japanese. Plan 5, the plan ultimately adopted with the addition of War Plan Orange, was a contingency for rapidly deploying American forces to Africa and Europe to defeat the Axis in concert with the Allied powers of Europe.
After Pearl Harbor, U.S. forces were deployed to both theatres as a reassurance to America's allies despite a 'Europe First' strategy developed in 1940. It was not until the buildup of forces for the invasion of Normandy that the majority of U.S. forces were in Europe. Eventually, over 75 percent of the American military was deployed to Europe, with only one quarter deployed to the Pacific.
Despite extensive planning, there were many developments that changed the course of the war in the Allies' favor. Germany's loss in the Battle of Britain caused the cancellation of Operation Sea Lion, the planned invasion of the British Isles. The dissolution of the Molotov-Ribbentrop Pact gave the Allies an advantage in Europe. Japan's insistence on destroying the U.S. Navy in a decisive battle led to decisions that allowed the U.S. to destroy more Japanese ships and be safer in their own movements. Planners did not foresee just how effective aircraft carriers would be in modern naval warfare, though they had correctly anticipated the prominence of submarines.
Ultimately, the version labeled 'Rainbow Plan 5' proved the right choice for an Allied victory in World War II.