U.S.–British Staff Conference (ABC–1)

Secret discussions between United States and British military staff members on American, British and Canadian (ABC) military coordination in the event of U.S. entry into World War II. The conference took place in Washington, D.C. from January 29, 1941 to March 27, 1941.

The US-British Staff Conference Report of 1941 established the general military principles, resources and deployment strategies for a joint Allied military strategy. The plan assumed that if the U.S. went to war with Germany, it would likely go to war with both Italy and Japan as well. The general principles of agreement stated that:

America's territorial interest was in the Western Hemisphere
The security of the British Commonwealth must be maintained in all circumstance including the retention of a Far East position
Security of sea communications between Allied Powers was essential

Offensive policies included:

The "early elimination" of Italy as an Axis partner
Support of neutrals and underground groups in resisting the Axis
Sustained air offensive to destroy Axis military power
Build-up of forces for the eventual offensive against Germany and the capture of positions from which to launch that offensive
The Atlantic and European areas was the "decisive theater" and as such would be the primary focus of US military efforts, although the "great importance" of the Middle East and Africa were noted.

Finally, if Japan entered the war, military strategy in the Far East would be defensive. Although the ABC-1 agreement was not a military alliance, it nevertheless signified that the United States was preparing to enter the war on the side of the Allied powers, that the United States would make every effort to maintain the security of the British Commonwealth, and that the United States military was modifying existing war plans (e.g. the RAINBOW plans) to incorporate military integration and cooperation with other nations.

Source

Watson, Mark Skinner. "U.S. Army in World War II, Chief of Staff: Pre-war Plans and Preparation," Chapter 12, Coordination with Britain.
Available online at: http://www.ibiblio.org/hyperwar/USA/USA-WD-Plans/