President Trump’s initiatives have frequently invoked national security — but in the absence of any actual threat to the nation. That may make a difference in the courts.

As profoundly disturbing as Mr. Trump’s sweeping and irresponsible violations of civil liberties have been, in one respect they are not exactly unprecedented. Previous presidents have deported people for their speech, targeted internal critics for blacklists, locked up foreign nationals without charge, invoked military authority at home and sought to muzzle the media. But there is one fundamental difference.

Virtually every time previous presidents took such extreme steps, they acted in response to at least a plausible threat to national security. This time, Mr. Trump has invoked emergency powers in the absence of any even plausible emergency. That fact simultaneously underscores how unhinged from reality the president’s actions are and provides reason for hope that they can be stopped.

Consider earlier periods of repression. The Alien and Sedition Acts of 1798 were enacted in the face of a potential war with France and fear of French sympathizers at home. Lincoln suspended habeas corpus during the Civil War, a threat to the very existence of the nation. In World War I, the government prosecuted those who spoke out against the war. The Palmer Raids of 1919-20, in which authorities rounded up and sought to deport thousands of foreign citizens for their political affiliations, were prompted by a coordinated series of anarchist bombings. World War II brought the internment of both Japanese nationals and U.S. citizens of Japanese descent, as well as the summary execution of German saboteurs caught on domestic soil.

In the McCarthy era there was perhaps the most widespread repression, but it was driven by concern that the Soviet Union sought to overthrow the United States by force — a fear that while greatly exaggerated was not entirely unfounded. And the George W. Bush administration’s deployment of torture, disappearances, so-called black sites and warrantless collection of phone data on Americans was, of course, a response to the terrorist attacks on 9/11.

In none of these cases did the threats justify the means adopted. They are all correctly seen, in retrospect, as severe overreactions and lamentable failures to live up to our ideals. The existence of such threats at least helps explain what happened, even if it does not make it right.