The Trump administration floated an idea in recent weeks: suspending habeas corpus.

Why? Because over and over again—including in the American Civil Liberties Union’s challenge against President Donald Trump’s illegal use of the Alien Enemies Act to deport people without due process—courts hearing habeas corpus cases have stopped the administration from carrying out massive violations of people’s constitutional rights. To get around this obstacle, administration officials now say they are looking into suspending habeas corpus altogether.

What is this arcane-sounding legal device? How is does it stand in the way of illegal government action?

Habeas corpus, known as the “Great Writ,” is a centuries-old legal instrument. The phrase is Latin for “you have the body.” At its core, it is a demand that the government bring any detainee before a court and explain why it has the authority to detain the person.

The U.S. Constitution enshrines this protection in Article I, Section 9, stating that the writ “shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it.” That’s a high bar, and for good reason: Habeas corpus has stood for centuries – even before the United States was founded – as a barrier against tyranny, It prevents kings, presidents, and prime ministers from locking people up without cause or review.

As Alexander Hamilton wrote, “[T]he practice of arbitrary imprisonments, have been, in all ages, the favorite and most formidable instruments of tyranny.” He therefore viewed the guarantee of habeas corpus perhaps “[a] greater securit[y] to liberty and republicanism than any [the Constitution] contains.” In fact, habeas corpus was one of the few individual rights in the Constitution before the Bill of Rights was adopted.

Habeas corpus ensures that no person — citizen or not — can be held by the government without the right to challenge their detention before a judge. It is a cornerstone of due process.

Among other things, it protects against:

• Indefinite detention without charge
• Imprisonment without a fair hearing
• Detention based on unlawful or discriminatory grounds
• Unlawful removal or transfer
• Government abuse of wartime or national security powers

Whether the government is detaining someone at Guantánamo Bay, in a Immigration and Customs Enforcement detention facility, or under a wartime statute like the Alien Enemies Act, habeas corpus is a constitutionally guaranteed pathway to a fair legal process.

President Trump is attempting to revive arcane laws to sidestep constitutional safeguards that prevent the detainment and deportation of individuals without providing their day in court.

His administration has:

• Invoked the 1798 Alien and Enemies Act to justify mass detentions and deportations to a torture prison in El Salvador.
• Locked up protesters for their protected speechand argued that they must sit in jail for years before seeing a federal judge.
• Deported people in violation of court orders and without hearings, notice, or access to legal counsel.
• Argued that these individuals have no right to challenge their detention.

But the courts have repeatedly found that habeas corpus guarantees a judicial forum to check these abuses and protect the rights of noncitizens.

Trump’s moves echo past abuses of power. In 2006, President George W. Bush and Congress enacted the Military Commissions Act (MCA), which sought to strip habeas rights for Guantánamo detainees. The Supreme Court struck it down in Boumediene v. Bush, reaffirming that habeas corpus cannot be bypassed simply by labeling someone an “enemy.”

President Trump repeatedly pushed that boundary during his first term — attempting to deny detained people access to the courts, defending the right of indefinite detention of noncitizens without meaningful judicial review, and even floating the idea of sending terrorism suspects to Guantánamo Bay. Now, his officials are going even further

To date, the Supreme Court has twice reaffirmed in the ACLU’s cases that anyone detained under the Alien and Enemies Act still has the right to file habeas petitions.

We have worked to vindicate that right — filing multiple habeas cases across the country one challenging unlawful detention under Trump’s sweeping executive order. Habeas corpus protection is what stands in the way of the government being able to arbitrarily take people from their communities in this country and send them to a notorious prison in El Salvador.

A president may claim they have the power to imprison people without charge or review, but justice demands a different answer. Habeas corpus thus stands as a crucial line of defense: for freedom and against abuse of power. And when that habeas corpus itself comes under attack, it’s up to all of us to defend it.