From the earliest days of the nation, U.S. and Vatican leaders have considered it worthwhile to keep diplomatic channels open, despite the sometimes rocky political paths involved. Through times when the Vatican has served as intermediary between sparring nations, to a diplomatic-channel offer from the United States — politely rejected — to find out who might have ordered an assassination attempt on Pope John Paul II, the relationship has often been beneficial to each side, whether the ties were formal or not. A fortunate bit of timing brought a refresher course in the history of U.S.-Vatican diplomacy to Washington May 28. It was the same day word got out about President Barack Obama’s nomination of theology professor Miguel Diaz to be the ninth U.S. ambassador to the Vatican.