Martial law is supposed to be a temporary state of military rule during an emergency like a natural disaster, a foreign invasion or a riot. But as history has shown, both authoritarian governments and democracies have routinely exploited martial law to suppress political opposition or hold onto power.

The term martial law is taken from the adjective "martial," which means pertaining to the military, and is derived from Mars—the ancient Roman god of war. Under martial law, many of the roles normally performed by civilian authorities are handed over to the military. That includes emergency response, policing and even the courts.

In theory, the laws of the land shouldn’t change under martial law, including constitutional protections and civil liberties. In practice, though, martial law is often used as legal cover to imprison people without a fair trial.

“The problem is that martial law is often a tool that strongmen use as a pretext,” says Stephen Vladeck, a law professor at Georgetown University. “And so the trick is to split the difference between the rare and exceptional cases where martial law is actually beneficial and the far more common cases where it's used to suppress civil liberties, infringe on our rights or overthrow our democratic institutions.”

Here are five notable times that martial law was declared, both in the United States and abroad.

Martial law makes sense during wartime, particularly when one-half of the country is at war with the other. Abraham Lincoln made extensive use of martial law during the Civil War, especially in regions of the country where local government was in disarray or couldn’t be trusted to enforce the laws of the Union. That included border states like Missouri, Kentucky and Indiana.

Lincoln issued proclamations authorizing the use of military tribunals instead of civilian courts to try suspected Confederate sympathizers. Proclamation 94, signed in 1862, extended the use of martial law to “all rebels and insurgents, their aiders and abettors, within the United States, and all persons discouraging volunteer enlistments, resisting militia draft or guilty of any disloyal practice affording aid and comfort to rebels against the authority of the United States.”

In a landmark Supreme Court decision handed down after the war, the justices rebuked the Union for overstepping its authority. In Ex parte Milligan (1866), the court ruled that military tribunals could only be used when no civilian courts were available. That wasn’t the case in Indiana, where the defendant, Lambden P. Milligan, was sentenced to death for disloyalty to the Union.

“The court went out of its way to say martial law is not appropriate when the civilian courts ‘are open and their process is unobstructed,’” says Vladeck. “That really drives home the limits on martial law. Which is that, when the civilian courts are open, when they're able to do their job, when they can issue writs of habeas corpus, that's pretty powerful evidence that there's no justification for martial law to continue.”

By 1933, Adolph Hitler and his Nazi Party had risen out of obscurity to win a share of Germany’s rule under a coalition government. Hitler rode a wave of populist German resentment with the failures of World War I and severe economic hardship, and blamed all of Germany’s problems on Jewish conspirators and communists.

On February 27, 1933, the German parliament building (Reichstag) in Berlin was set ablaze. German police arrested a Dutch laborer who allegedly confessed that the arson was meant to inspire a communist uprising. Hitler seized on the Reichstag fire to stoke fears of a communist takeover and convinced parliament to institute martial law.

The declaration, known as the Reichstag Fire Decree, went far beyond the typical scope of martial law. Instead of just placing civil authorities under military control, Hitler’s law suspended nearly all civil liberties for German citizens: freedom of speech, freedom of the press, the right to assemble, etc. The Nazi military police, known as the Sturmabteilung or “storm troopers,” rounded up thousands of suspected communists, who were imprisoned without trial.

The “emergency” conditions in 1933 that prompted the Reichstag Fire Decree paved the way for an all-out Nazi dictatorship that was only ended by Germany’s defeat in World War II.

In the chaos following the Japanese bombing of Pearl Harbor on December 7, 1941, all of the Hawaiian Islands were placed under martial law. But instead of being a temporary measure until the civilian government was restored, martial law remained in effect in Hawaii for the duration of World War II.

Hawaii wasn’t a state yet in the 1940s, but it was still a U.S. territory with all of the rights and privileges under the Constitution. Under martial law, however, Hawaii’s citizens—including large numbers of people of Japanese descent—were routinely denied their basic civil rights, including the right to a fair trial.

During World War II in Hawaii, the military performed all of the roles of civilian government, from collecting trash to hearing criminal cases that had nothing to do with military threats. In one case, a civilian stockbroker named Harry White was arrested and convicted of embezzlement by a military court, not a jury.

White and others took their cases to the Supreme Court, which ruled in Duncan v. Kahanamoku (1946) that martial law was applied unconstitutionally in Hawaii. After the initial military threat was over, Hawaiian citizens should have been tried by civilian authorities, not the military.

“The lesson there is that martial law may be required in the immediate aftermath of some kind of natural or man-made crisis,” says Vladeck, “but every hour that passes, every day that passes, every moment where civilian authority is able get back on its feet would be an increasing argument against its continuance.”

In 1972, Ferdinand Marcos was nearing the end of his second and final term as president of the Philippines. Marcos and his wife Imelda—a former beauty queen—maintained a lavish lifestyle while most Filipinos lived in poverty. Desperate to hold onto power, Marcos announced his plan to stay on as prime minister of a new parliamentary government. When protests erupted, Marcos blamed it on a communist plot and declared martial law. It lasted for nearly a decade.

Under martial law, any opposition to the Marcos regime was brutally suppressed. Tens of thousands of Filipinos—students, journalists, academics and religious leaders—were arbitrarily arrested, detained and tortured. Untold numbers were executed, including the key opposition leader, Benigno Aquino Jr. After martial law was finally lifted in 1981, Marcos lost an election to Aquino’s widow, but claimed it was rigged. When massive, countrywide protests erupted, Marcos was airlifted to Guam by American officials. He died in exile in Hawaii.

In April of 1989, student protestors took to Beijing’s Tiananmen Square calling for change. This was a time when communism’s grip was weakening in the Soviet Union and its satellite states, and the students were hoping that China would follow suit with political and economic reforms. As the weeks wore on, upwards of a million protestors flooded into Tiananmen Square, which drew unwanted attention from the international media.

On May 20, the Chinese Premier Li Peng addressed the nation and declared a state of martial law in Beijing. Li assured the public that he was only sending in the People’s Liberation Army (PLA) to restore order and maintain public security, not to crack down on the protests.

“The PLA troops' arrival is definitely not aimed at dealing with the students,” said Li. “It is hoped that all trades and professions, people in various circles, and particularly people of the various democratic parties will support the PLA troops for their action to safeguard the capital and maintain public security. I hope that you will give them your full understanding and support.”

PLA troops arrived by the truckload and were met with largely peaceful resistance by Beijing’s citizens, who encouraged the soldiers to put down their weapons and join the cause. Instead, the PLA initiated a brutal crackdown. On the nights of June 3 and 4, Chinese soldiers stormed Tiananmen Square with tanks and machine guns. The death toll in Beijing will never fully be known, but likely reached the thousands.