Signalgate was the national security adviser’s most glaring mistake. But his problems ran deeper. After Michael Waltz, the national security adviser, inadvertently included The Atlantic ’s editor in chief in a group chat about military attack plans on the Signal messaging app, he found himself on very thin ice with his boss.

Mike Waltz, the national security adviser for President Donald Trump, is seen in photos using Signal during a Cabinet meeting on April 30.

President Trump recently discouraged the use of the Signal app after controversy over a group chat of administration officials detailed military plans.