What Is Habeas Corpus?

Habeas corpus has existed longer than the United States and is relatively simple to understand when broken down: a legal instrument “that requires that the government provide a public reason for detaining and imprisoning people,” U.S. Senator Maggie Hassan, a former attorney, said in May 2025. Its fundamental purpose is to allow people to challenge unlawful detention. Article I, section 9, clause 2 of the U.S. Constitution, known as the Suspension Clause, says the privilege of habeas corpus "shall not be suspended, unless in Cases of Rebellion or Invasion the public Safety may require it.” 

Modern-day habeas is mostly used in criminal post-conviction cases when incarcerated people believe their constitutional rights were violated, such as during their trial or sentence phases.

Yearly, about 12,000 writ of habeas corpus cases are filed, though the success rate is just over 10% in capital cases and less than 1% in others.

Historical habeas corpus cases usually revolved around a president attempting to suspend its protections. Abraham Lincoln used the Suspension Clause during the Civil War against traitors in Maryland. Under the suspension, John Merryman was arrested and detained at Fort McHenry outside of Baltimore. Merryman petitioned Supreme Court Justice Roger Taney for a writ of habeas corpus; Taney sided with Merryman.

The U.S. Supreme Court decided detainees at Guantanamo Bay who were held after the 9/11 attacks were allowed to use habeas corpus to challenge their detentions. Some detainees were successfully released under the writ, including five Algerian men who were freed when federal Judge Richard J. Leon said the government’s secret evidence from an “unnamed source” in the case had been too weak to justify their detentions. 

Today, President Donald Trump has attempted to suspend habeas corpus by invoking the Alien Enemies Act, which allows a president to detain or deport migrants from enemy nations during declared war, invasion or a threatened invasion. Venezuelan immigrants accused of being gang members have successfully filed habeas corpus petitions across the country, arguing that the Alien Enemies Act is not a legal basis for their detention. 

Every court that has considered the Alien Enemies Act has ruled in favor of the immigrants, ultimately deciding that they were not gang members and Trump had improperly invoked the law during peacetime.

Other petitions have been filed to return immigrants who were already deported under the act, including Kilmar Abrego Garcia, who was held without legal basis in a Salvadoran prison after the Trump administration deported him from Maryland.