Under the Treaty of Versailles, the German emperor was supposed to be tried as a war criminal. Why wasn't he? The accusations were explosive: a head of state had not only begun an illegal war but egged his troops on to a series of horrific atrocities that left thousands dead and an entire continent in ruins. By then, the accused was one of history’s most hated and debated figures, a monarch known for making erratic decisions and doubling down on his sometimes inexplicable actions. There was just one problem: The accused, Wilhelm II of Germany, couldn’t testify. The accused had...