During the 1920s and 1930s, the United States military Joint Army and Navy Board developed a number of color-coded war plans to outline potential U.S. strategies for a variety of hypothetical war scenarios. The plans, developed by the Joint Planning Committee (which later became the Joint Chiefs of Staff), were officially withdrawn in 1939 in favor of five Rainbow Plans developed to meet the threat of a two-ocean war against multiple enemies.
The following war plans were drawn up:
War Plan Black-A plan for war with Germany. The best-known version of Black was conceived as a contingency plan during World War I, in case France fell and the Germans attempted to seize French possessions in the Caribbean Sea, or launch an attack on the eastern seaboard.
War Plan Grey-There were two War Plans named Gray. The first dealt with Central America and the Caribbean, and the second dealt with invading the Portuguese Azores.
War Plan Brown-Dealt with an uprising in the Philippines.
War Plan Tan-Intervention in Cuba.
War Plan Red-Plan for the United Kingdom (with sub variants Crimson, Scarlet, Ruby, Garnet, and Emerald for British dominions)
War Plan Orange-Plan for Japan.
War Plan Red-Orange-Considered a two-front war with the United States (Blue) opposing Japan (Orange) and the British Empire (Red) simultaneously. Ultimately this analysis led to the understanding that the United States didn't have the resources to fight a two front war, and it would make sense to focus on one front, probably in the Atlantic. Ultimately this was the decision made in the Plan Dog memo.
War Plan Yellow-Dealt with war in China—specifically, anticipating a repeat of the Boxer Uprising (1899–1901). War Plan Yellow would deploy the US army in coalition with other imperial forces to suppress indigenous discontent in the Shanghai International Settlement and Beijing Legation Quarter, with chemical weapons if necessary.
War Plan Gold-Involved war with France, and/or France's Caribbean colonies.
War Plan Green-Involved war with Mexico or what was known as "Mexican Domestic Intervention" in order to defeat rebel forces and establish a pro-American government. War Plan Green was officially canceled in 1946.
War Plan Indigo-Involved an occupation of Iceland. In 1941, while Denmark was under German occupation, the US actually did occupy Iceland, relieving British units during the Battle of the Atlantic.
War Plan Purple-Dealt with invading a South American republic.
War Plan Violet-Covered Latin America.
War Plan White-Dealt with a domestic uprising in the US, and later evolved to Operation Garden Plot, the general US military plan for civil disturbances and peaceful protests. Parts of War Plan White were used to deal with the Bonus Expeditionary Force in 1932. Communist insurgents were considered the most likely threat by the authors of War Plan White.
War Plan Blue-Covered defensive plans and preparations that the United States should take in times of peace.
*All of these are from Wikipedia*
Anyhow, pick which one would be the best and most likely one to occur and explain what effects it would have up to present day and what it would mean for the U.S going into WW2.