American military plans for a war with the British Empire, first discussed in 1919, have received varied treatment since their declassification. the most common theme among historians in their appraisals of WAR PLAN RED is that of an oddity. Lack of a detailed study of Anglo-American relations in the immediate post-First World War years makes a right understanding of the difficult relationship between the United States and Britain after the War problematic. As a result of divergent aims and policies, the United States and Great Britain did not find the diplomatic and social unity so many on both sides of the Atlantic aspired to during and immediately after the First World War. Instead, United States’ civil and military organizations came to see the British Empire as a fierce and potentially dangerous rival, worthy of suspicion, and planned accordingly. Less than a year after the end of the War, internal debates and notes discussed and circulated between the most influential members of the United States Government, coalesced around a premise that became the rationale for WAR PLAN RED. Ample evidence reveals that contrary to the common narrative of “Anglo-American” and “Atlanticist” historians of the past century, the First World War did not forge a new union of spirit between the English-speaking nations. the experiences of the War, instead, engendered American antipathy for the British Empire. Economic and military advisers feared that the British might use their naval power to check American expansion, as they believed it did during the then recent conflict. the first full year of peace witnessed the beginnings of what became WAR PLAN RED. the foundational elements of America’s war plan against the British Empire emerged in reaction to the events of the day. Planners saw Britain as a potentially hostile nation, which might regard the United States’ rise in strength as a threatening challenge to Britain’s historic economic and maritime supremacy.