In what may be the final volume of a tetralogy covering U.S. activity in and around Iraq over the past three decades, Michael Gordon’s “Degrade and Destroy” combines Washington decision-making with battlefield reporting in ways that few other writers can manage. This account of America’s war against the Islamic State is Mr. Gordon’s first without co-author Bernard Trainor, who died in 2018, but it equals its forerunners in quality. While daily press reporting strains to draw overbroad conclusions from insufficient data, Mr. Gordon, a national security correspondent for the Journal, maximizes history and minimizes judgments. He presents his analysis, of course, but it’s always moored in reality.