Um, yeah, in fact, the "yanks" did participate in World War I to, I would assert, a rather significant extent. Nearly 5 million Americans served in WWI; more than 116,000 Americans died in that war. The U.S. Veterans Day is on November 11 in remembrance of the armistice! I could direct you to any number of history books, but somehow I'm getting the feeling that's not what you're looking for.