I've heard that there weren't any specifically allied tied made until WWI. Britain supplied the Confederacy with supplies during the Civil War (being essentially their only source for industrial resources and weapons), and this was not appreciated by the Union. There clearly were some positive relations formed in the fifty year gap between two wars, seeing as how the U.S. fed supplies to the U.K. long before deploying any troops (in WWI), but U.S. involvement was more concerned with stopping the Central Powers than aiding Britain.