Two days after the missile strike, Trump sent Congress a notice that he had ordered it and that he had the “constitutional authority” to do so.

Sen. Tim Kaine of Virginia and Rep. Adam Schiff of California sent a stern letter to the White House on Tuesday, warning that Trump could be setting a dangerous precedent for conducting pre-emptive strikes and risking war with major powers, while cutting Congress out of the picture.

Two Democratic members of Congress are demanding that Trump offer some sort of legal justification beyond off-the-cuff remarks from administration officials.

Nearly three weeks after ordering a cruise missile attack against one of Syrian President Bashar Al-Assad’s airfields, Donald Trump has yet to explain how that was legal without congressional authorization.

Kaine and Schiff wrote: “These assertions of authority do not provide Congress with the information it needs to exercise our constitutional responsibilities. Nor do they provide comfort to a public that fears deeper involvement in a horrific civil war at a time when the U.S. troop presence in the region is already increasing. The legal justification for an attack on the Syrian government is not an afterthought, but rather a first order consideration, and something that is vital for the American people to understand at the outset.”

The authors also expressed concern that the Trump administration might take pre-emptive strikes against North Korea without consulting Congress.

“While the President has the authority to use force to defend our service members and allies from an imminent threat, a preemptive strikes could easily spiral into a full-fledged war with a nuclear armed adversary,” Kaine and Schiff wrote. “It is precisely because the decision to go to war is such a momentous one for any nation that the Constitution provides Congress alone with the power to declare war.”

Several administration officials have defended the Syria attack by saying that it is in U.S. interests to deter future chemical weapons strikes. But that is not the same thing as saying the attack was conducted in self-defense or in response to an imminent threat, which would at least resemble past presidents’ justifications for not consulting Congress.

Asked why Trump did not seek congressional authorization, White House Press Secretary Sean Spicer said that “when it’s in the national interest of the country the president has the full authority to act.”

But that explanation did not satisfy a number of legal scholars, who argued that Trump’s strikes were illegal without authorization. “President Trump has no constitutional authority to unilaterally commit the nation to war against Syria, which is the effect of launching cruise missiles against Syria,” wrote Louis Fisher, a scholar in residence at the Constitution Project.

Back in 2013, when President Obama was considering strikes against Assad in retaliation for using chemical weapons, Trump tweeted that Obama “must get Congressional approval,” and that it would be a “big mistake if he does not.”

Obama did seek congressional authorization — and it was denied, although he consistently maintained that he did not need it. At the time, Jack Goldsmith, the former head of the Justice Department’s Office of Legal Counsel during the Bush administration, wrote that Obama’s reasoning placed “no limit at all on the president’s ability to use significant military force unilaterally.”

Congress in 2001 passed a resolution allowing the president to use “all necessary and appropriate force against those nations, organizations, or persons” that were involved in the 9/11 terror attacks. And since then, the legislative branch has consistently ceded its authority in deciding to go to war. Bush and Obama have used the 2001 resolution to justify mass surveillance, bombing campaigns from Somalia to Pakistan, and prison operations from Afghanistan to Guantánamo Bay, Cuba.

Top photo: This DigitalGlobe satellite image from April 7, 2016, shows the Shayrat Air Base outside of Homs, Syria, following the U.S. air strike.