Ukraine crisis: What is martial law?

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Mike Tobin on the streets of Lviv amid Russian military invasion of Ukraine

Fox News' Mike Tobin reporting in Ukraine on how citizens are dealing with escalation from Russian troops

Martial law is a term that describes the government order of replacing civil rule with the military in a time of war or emergency.

The topic of martial law has resurfaced as a result of the escalating conflict in Ukraine, where Russia launched a wide-ranging attack on Thursday.

Russia unleashed airstrikes on the eastern European country, hitting cities and bases as civilians piled into cars and trains to flee. 

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Ukrainian Military Forces servicemen block a road in the so-called government quarter in Kyiv on February 24, 2022, as Russia's ground forces invaded Ukraine from several directions today, encircling the country within hours of Russian President announcing his decision to launch an assault. (Sergei Supinsky/AFP via Getty Images)

Ukraine’s government said Russian tanks and troops rolled across the border in a "full-scale war" that could rewrite the geopolitical order and whose fallout already reverberated around the world.

Ukrainian President Volodymyr Zelenskyy cut diplomatic ties with Moscow and declared martial law.

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"As of today, our countries are on different sides of world history," Zelenskyy tweeted. "Russia has embarked on a path of evil, but Ukraine is defending itself and won’t give up its freedom."

Martial law is the imposition of military control over civilians, usually during a time of war. Civilian courts are shut down, and military justice prevails.

It’s been declared several times throughout American history. Hawaii, when it was a U.S. territory, was placed under martial law for much of World War II. President Abraham Lincoln declared martial law during the Civil War.

An article shared by the U.S. Department of Justice’s Office of Justice Programs, titled "Martial Law in Times of Civil Disorder," describes how "the military commander of an area or country has unlimited authority to make and enforce laws" when martial law has taken effect. 

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"Martial law is justified when civilian authority has ceased to function, is completely absent, or has become ineffective," the publication states. "Further, martial law suspends all existing laws, as well as civil authority and the ordinary administration of justice."

The Associated Press contributed to this report.