You do realize that most nations have war plans on how to proceed with both offensive and defensive wars with just about any country with a significant military force, right? War Plan Orange detailed plans on a war with Japan, and was devised prior to WWII... War Plan Red-Orange was a two front war with Japan
and Britain. In fact, read this:
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 Gray
There were two War Plans named Gray. The first Central America[10] 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 Great Britain (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, the defense of Beijing and relief of Shanghai during the Second Sino-Japanese War.
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.
http://en.wikipedia.org/wiki/United_States_color-coded_war_plans
Plus, given the fact that the United States exists because of a war with Britain, and there had already been a subsequent conflict in the form of the War of 1812; planning for war with Britain was a case of covering their asses just in case it was necessary. You've ignored the fact that War Plan Red was viewed as "unlikely" and "highly improbable"... not to mention the fact that the "special relationship" didn't actually begin until WWII.