In 1955 the Supreme Court ruled that veterans of the U.S. armed forces could not be court-martialed for overseas crimes that were not detected until after they had left military service. Territorial limitations placed such acts beyond the jurisdiction of civilian courts, and there was no other American court in which they could be adjudicated. As a result, a jurisdictional gap emerged that for decades exempted former troops from prosecution for war crimes. "This was not merely a theoretical possibility," Patrick Hagopian writes. Over a dozen former soldiers who participated in the My Lai...