Here is a very interesting essay on religion and American foreign policy by Walter Russell Mead in Foreign Affairs.
He makes an important distinction by dividing religious American Protestants into three groups: liberals, fundamentalists, and evangelicals. Then he focuses on the evangelicals, who are growing in numbers and in their interest in foreign policy. Walter is the son of an Episcopalian minister who was assigned to South Carolina during the days of desegregation and later worked at the National Cathedral in Washington; his parents now live across the street from me. Walter is an appreciator and a describer, not an advocate and a decrier. His splendid book Special Providence has brilliant sections on the role of religion in American foreign policy over the years. Read the whole thing: the article and then the whole book.