"The Army is a broadsword, not a scalpel. Trust me, senator, you do not want the Army in an American city."
--General William Devereaux (The Siege)
During the Civil War, President Abraham Lincoln dealt with those in Union states voicing opposition to the war by arresting dissenters under auspices of martial law. Thousands were jailed without writs of habeas corpus and tried before military commissions rather than civil courts.
One such dissenter, an Indiana lawyer name Lambdin Milligan, petitioned the federal circuit court in Indianapolis. His case, which became known as Ex parte Milligan, was passed to the Supreme Court. The primary question was whether Lincoln's military tribunals against dissenters were legal.
The Court unanimously rebuffed Lincoln's efforts to cut constitutional corners. The Court wrote:
"The Constitution of the United States is a law for rulers and people, equally in war and peace, and covers with the shield of its protection all classes of men, at all times, and in all circumstances. No doctrine, involving more pernicious circumstances, was ever invented by the wit of a man than that any of its provisions can be suspended during any of the great exigencies of government. Such a doctrine leads directly to anarchy or despotism."
Via its ruling in Milligan, the Court tells us that the Constitution is always on. No exceptions.