In early June, President Trump threatened to “deploy the United States military and quickly solve the problem.” A day later, Sen. Tom Cotton (R-Ark.) in a New York Times op-ed suggested that the federal government invoke an old and obscure law — the Insurrection Act — to use the military to “restore order” in those cities in which the protests arising from the killing of George Floyd had turned violent.

So what is the Insurrection Act? This is the common name for statutes enacted by Congress — in 1792, 1795, 1807, 1861 and 1871 — governing when the U.S. president can use military force in response to an array of domestic crises. Here’s what the president can and cannot do.

What is the Insurrection Act of 1807?

The origins of the Insurrection Act of 1807 begin with Aaron Burr. As the story goes, after being jettisoned as Thomas Jefferson’s vice president in the 1804 elections, Burr spent the better part of 1805 and early 1806 allegedly fomenting rebellion or insurrection in what was then the American “southwest” and Spanish-controlled Mexico. Whether Burr was attempting to form a new country or to build a force that could overwhelm the U.S. government remains unclear.

In response, Jefferson introduced — and, on its very last day in session, the Ninth Congress passed — a one-sentence statute:

“In all cases of insurrection, or obstruction to the laws, either of the United States, or of any individual state or territory, where it is lawful for the President of the United States to call forth the militia for the purpose of suppressing such insurrection, or of causing the laws to be duly executed, it shall be lawful for him to employ, for the same purposes, such part of the land or naval force of the United States, as shall be judged necessary, having first observed all the prerequisites of the law in that respect.”

Because of the powers Congress had already given to state militias, in one stroke of the pen, Jefferson, who is hardly known as a supporter of broad executive power, signed into law a statute that has expanded presidential power in ways that, as legal scholars argue, seem to be out of step with the intentions of the founding generation.

The Constitution, Congress and the militia

The Constitution’s drafters were simultaneously wary of both an impotent central government (like the one that had been powerless to put down Shays’ Rebellion in 1786) and a standing army. To split the difference, Article I, Section 8 gave Congress the power to provide for the “calling forth of the militia” for domestic emergencies — “to execute the laws of the union, suppress insurrections and repel invasions.” And the choice to give that responsibility to the militia, rather than the army, was deliberate. Because the militia was composed of citizen-soldiers from the same place as the trouble to which they were responding, the idea was that it would be less prone to abuse by a tyrannical executive in a faraway capital.

Relying upon these constitutional provisions, Congress first temporarily authorized the use of the militia to respond to domestic emergencies in May 1792. This test statute authorized the president to call out state militias in the circumstances the Constitution contemplated, but imposed strict limits on his ability to use such power. Any use of an out-of-state militia expired 30 days after the beginning of the next congressional session, and a federal judge or Supreme Court justice had to sign off on the president’s factual determination that circumstances warranted a resort to military force.

President George Washington followed these procedures to the letter in putting down the Whiskey Rebellion in 1794. But when Congress made the delegation permanent in 1795, it removed the judicial certification requirement, giving the president authority to decide when to call in state militias to respond to domestic crises.

The 1861 and 1871 amendments

Congress would amend what we know today as the Insurrection Act twice — in 1861, on the eve of the Civil War, to settle beyond peradventure its applicability to the seceding Southern states; and in 1871, amid Reconstruction, to make clear that enforcing the new constitutional amendments — and statutes enacted to enforce them — was also part of the military’s authority.

So where does the Insurrection Act stand in 2020? The statute created three separate authorities: The first, codified at 10 U.S.C. § 251, traces all the way back to the 1792 act and allows the president to use troops when requested by the state in which the crisis is taking place. The second, codified at 10 U.S.C. § 252, traces to the 1861 act and allows the president to use troops, even without a state request. And 10 U.S.C. § 253, derived from the 1871 act, likewise allows the president to send troops when “any insurrection, domestic violence, unlawful combination, or conspiracy” prevents enforcement of state laws in a manner that deprives residents of that state of their federal constitutional rights.

The statute requires the president to issue a formal proclamation before invoking any of these powers. But once he does, there is no intrinsic limit on how long he can use federal troops to enforce federal law.

The Insurrection Act may not be constitutional

It’s been 28 years since a president last used federal regulars on U.S. soil for domestic law enforcement, when President George H.W. Bush sent troops into Los Angeles amid the Rodney King riots, at the request of Gov. Pete Wilson (R). Invoking the statute has also been unpopular — that’s why President George W. Bush refused to do so without a state request in the aftermath of Hurricane Katrina.

As the plain text and history of the act demonstrate, Trump would need to issue a formal proclamation before invoking any of these powers. Even then, any use of the act would raise serious constitutional questions that the Supreme Court has never answered — all the more so if the president claims a need to restore order in a state that has not requested help.

Steve Vladeck (@steve_vladeck) is the A. Dalton Cross Professor in Law at the University of Texas School of Law and co-host of “The National Security Law Podcast.”