Germany has long occupied an exceptionally comfortable place in the world. It has an export-dependent economy, selling its cars and machines far and wide — and many tanks and submarines, as one of the world’s largest arms exporters. But when it comes to countering perceived security threats — whether the Islamic State or Putin — it has allowed allies to take the lead. German leaders sent troops to Afghanistan but largely avoided referring to it as a “war,” even as German soldiers engaged in ground combat there for the first time since World War II. Germany’s aversion to military power...