I used to be Catholic, and he was not my favorite pope by any means, but here ya go from the NY Times in 2005 -
Enrollment in the Hitler Youth was mandatory for any high school age student. After that, he served for a time in an antiaircraft unit that guarded a BMW plant outside Munich -- and there are photographs that show the young Ratzinger in the paramilitary uniform of what were called the flak units, composed of under-age soldiers assigned to antiaircraft guns.
But historians and Jewish groups agree that the pope's wartime record, which was very common to young men of his generation, has little if any significance today and certainly suggests no sympathy for the Nazis, then or now.
It is true that by an accident of history, Benedict XVI is a pope who once wore a Wehrmacht uniform. But as chief adviser to Pope John Paul II in matters of doctrine, he was, far more importantly, a central figure in one of the late pope's most highly publicized gestures -- apologizing for the role that Catholics played in the Holocaust.