Aaron Bushnell’s death by self-immolation in front of the Israeli Embassy in Washington last month has provoked nationwide soul-searching about the war in Gaza. For the U.S. government though, the airman’s death excites a different kind of search: for so-called extremists, particularly left-wing ones. Last Wednesday, Sen. Tom Cotton, R-Ark., former Army officer and a member of the Senate Armed Services Committee, sent a letter to Defense Secretary Lloyd Austin asking why and how the Pentagon could tolerate an airman like Bushnell in its ranks. Calling his death “an act of horrific violence” that was “in support of a terrorist...