Crises offer these would-be authoritarians an escape from constitutional shackles. National emergencies — especially wars or major terrorist attacks — do three things for such leaders. First, they build public support. Security crises typically produce a rally-round-the-flag effect in which presidential approval soars. Citizens are more likely to tolerate — and even support — authoritarian power grabs when they fear for their safety. Second, security crises silence opponents, since criticism can be viewed as disloyal or unpatriotic. Finally, security crises loosen normal constitutional constraints. Fearful of putting national security at risk, judges and legislative leaders generally defer to the executive.

National emergencies can threaten the constitutional balance even under democratically minded presidents like Abraham Lincoln and Franklin Roosevelt. But they can be fatal under would-be autocrats, for they provide a seemingly legitimate (and often popular) justification for concentrating power and eviscerating rights. Hitler’s authoritarian response to the 1933 Reichstag fire is the most prominent example, but there are many others. In Peru, a Maoist insurgency and economic crisis enabled Mr. Fujimori to dissolve the Constitution and Congress in 1992 ; in Russia, a series of deadly apartment bombings in 1999 — allegedly by Chechen terrorists — triggered a surge of public support for Mr. Putin, who was then the prime minister, which allowed him to crack down on critics and consolidate his power; and in Turkey, a series of terrorist attacks in 2015, along with a failed 2016 coup attempt, allowed Mr. Erdogan to tighten his grip via a two-year state of emergency.

Crises present such great opportunities for concentrating power that would-be autocrats often manufacture them. In 1937, President Vargas of Brazil, resisting term limits that would force him to leave office the next year, used the “discovery” of a communist plot (the so-called Cohen Plan, later revealed to be a fabrication) to dissolve the Constitution and establish a dictatorship.

Similarly, President Marcos of the Philippines did not want to step aside when his second term expired in 1973. But he needed a reason to subvert constitutional checks. An opportunity arrived in 1972 , when a series of explosions rocked Manila. Following an apparent assassination attempt on his defense secretary, Marcos, blaming communist terrorists, declared martial law and established a dictatorship. This crisis, too, was largely fabricated: The bombings are believed to have been carried out by government forces and the assassination attempt was staged. The “communist menace” that Marcos used to justify martial law amounted to several dozen insurgents.

Although President Trump operates in a different political environment, his behavior, particularly since the November midterm elections, betrays similar autocratic instincts. The president manifestly lacks the patience or negotiating skills needed to deal with divided government. His response to Democratic control of the House of Representatives has been a refusal to compromise and, more dangerously, a refusal to lose. Unlike Presidents Clinton and Bush, who conceded defeat when it became clear that their initiatives lacked legislative support, Mr. Trump has refused to accept the failure of his border wall project. Unable to obtain the necessary votes in Congress, the president recklessly forced a government shutdown. When that didn’t get him his wall, he moved to circumvent Congress altogether by inventing — if not yet declaring — a national emergency. In his Oval Office speech on Tuesday, he used the word “crisis” six times in eight minutes. That is how autocrats respond to legislative opposition. Following in the tradition of Vargas and Marcos, Mr. Trump fabricated a security threat to make the case for bypassing Congress.