What do Hillary Clinton, Mike Huckabee, John McCain and Barack Obama have in common, besides wanting to be the next commander in chief? They are all of Scots-Irish descent, an ethnic and cultural lineage that has produced more presidents and military leaders than any other. "Fascinating, but not surprising," says U.S. Sen. Jim Webb, D-Va. The former naval secretary and decorated Marine should know: He catalogued the migration and cultural influence of the Scots-Irish in America in his book "Born Fighting." They have "always had the tradition of being in leadership positions, whether it is the military or in politics,"...