Future generations may mark today as one of the truly dark days in American history, a history that may soon take an even more ominous turn. President Trump’s sudden firing of FBI Director James Comey is a matter that should deeply concern every American, regardless of party, partisan politics or ideological leanings. The independence of our law enforcement is at the bedrock of our democracy. That independence, already grievously shaken under the brief tenure of Attorney General Jeff Sessions, is now shattered by uncertainty. The firing of an FBI Director is always a very serious matter in normal times. But...