In the Appalachian Mountains, writes James Webb in Born Fighting: How the Scots-Irish Shaped America, “American flags are frequent, on the trucks and in the yards and on the porches. America got bombed and mountain people don’t forget, even if it happened in New York and Washington, because when it comes to fighting wars, mountain people have always been among the first to go.” In chronicling the Scots-Irish, Webb writes of the people who made him and, he argues persuasively, America what both are today. It is a close call on which benefited the most from that lineage. Webb is...