One of the illuminating and sobering tasks that an historian can perform for a society is to look back at what was intended by, say, a nation’s founding fathers and compare that to what ended up happening over the years. Michael Beschloss does this expertly and sweepingly in Presidents of War (Crown, $35), which looks at how, despite a Constitution intended to make it difficult for a president to lead America into war—indeed, the sole power to declare war was given to Congress—presidents have gotten us embroiled in wars roughly once in every generation. Beschloss’s landmark book examines the experiences of eight presidents from the War of 1812 through the Mexican War, the Civil War, the Spanish-American War, World Wars I and II, Korea, and Vietnam. It also includes an early chapter on the skillful efforts of a ninth president, Thomas Jefferson, who kept the United States out of war.