The USS Porter launches a missile in the Mediterranean Sea on April 7, 2017. The U.S. military launched at least 50 missiles targeting an airfield in Syria in response to the Syrian military's alleged use of chemical weapons days earlier. (Ford Williams/Navy/European Pressphoto Agency)

Correction: The headline on an earlier version of this story incorrectly described the Justice Department’s advice. It did not tell President Trump that the airstrikes on Syria were “in the national interest”; instead, it found that the strikes were legal because the president had made a reasonable determination that they were in the national interest.

Before President Trump authorized U.S. airstrikes on three Syrian military sites in April, the Justice Department advised him that the strikes were legal because he had “reasonably” determined they were “in the national interest” and would not constitute a war, according to a department document released Friday.

In a broad interpretation of presidential powers, the Office of Legal Counsel (OLC) told Trump that he did not need congressional authorization. It cited earlier legal approvals of numerous similar actions by previous presidents, including a 2011 OLC opinion advising President Barack Obama that a “national interest” determination was sufficient for him to launch military action in Libya.

Then and now, government lawyers also concluded that no congressional involvement was necessary, because “the anticipated hostilities would not rise to the level of a war in the constitutional sense,” the new document said.

Lawmakers complained, then and now, that such opinions sidestep the power to declare war that the Constitution gives solely to Congress.

The new OLC opinion, dated Thursday, expands on a shorter judgment requested and delivered to Trump before the April aircraft and cruise-missile strikes, conducted along with France and Britain. They were launched, the three governments said, to punish an alleged chemical weapons attack on civilians by the armed forces of Syrian President Bashar al-Assad.

Sen. Tim Kaine (D-Va.) called the OLC reasoning “nonsense.”

“Is there any doubt that America would view a foreign nation firing missiles at targets on American soil as an act of war?” Kaine asked in a statement.

“The ludicrous claim that this President can magically assert ‘national interest’ and redefine war to exclude missile attacks and thereby bypass Congress should alarm us all,” he said. “This is further proof that Congress must finally take back its authority when it comes to war.”

Kaine, who has long been outspoken on the issue, made a similar argument after Trump ordered an earlier strike in Syria last year, also following alleged chemical weapons attacks on civilians. Although Republicans have been less exercised about Trump’s use of force without consulting lawmakers, many objected on the same grounds to Obama’s 2011 bombing of Moammar Gaddafi’s forces in Libya.

In 2013, Obama threatened strikes against Syrian military forces for chemical weapons use, claiming he had authority to carry them out “without specific congressional authorization.” After Congress indicated it disapproved, however, he decided not to do it.

After Vietnam, an undeclared war, Congress passed the War Powers Resolution, requiring congressional consent if U.S. forces are involved in “hostilities” overseas for more than 60 days. But the president, from the country’s founding, has had the authority, as commander in chief, to use force without asking, if it is required to defend the nation.

The “national interest” justification, however, is relatively recent. The new OLC opinion is “a far deeper reflection of just how broadly the Executive Branch, in general, construes its unilateral war powers today,” University of Texas law professor Stephen I. Vladeck wrote Friday on the Just Security website.

Harvard Law School professor Jack Goldsmith, who served as assistant attorney general in the OLC and special counsel to the Defense Department under President George W. Bush, noted that the new Justice Department document also outlines an expanded definition of “national interest” that again dates to the Obama administration.

The OLC opinion, signed by Assistant Attorney General Steven A. Engel of the OLC, refers to a previously unpublished Obama-era opinion that approves “an interest in mitigating humanitarian disaster as a basis for unilateral presidential force,” Goldsmith wrote Friday on the Lawfare blog, even if no Americans are affected by it.

“The Justice Department now officially and publicly believes the president can use significant air power, without congressional authorization [on the] grounds of humanitarian interventions and deterrence of the use of chemical weapons,” Goldsmith wrote.