The recent arrest of a senior German intelligence officer accused of spying for Russia is a timely reminder of the degree to which the Russian secret services may have penetrated Western institutions and of how much they know about Western capabilities and intentions. Carsten L., the German officer with “access to a trove of top-secret information about the war in Ukraine, as well as knowledge of how it was collected by the U.S. and its allies,” is only the most recent entry on a long list of alleged Russian spies. In October 2021, a few months before Russia’s invasion of...