In a 22-page legal opinion disclosed late Thursday, the Justice Department’s Office of Legal Counsel declared that Mr. Trump could lawfully and unilaterally direct airstrikes targeting Syria installations because he determined that doing so would be in the national interest, and because the attack would carry little risk of escalation.

“Given the absence of ground troops, the limited mission and time frame and the efforts to avoid escalation, the anticipated nature, scope and duration of these airstrikes did not rise to the level of a ‘war’ for constitutional purposes,” wrote Steven A. Engel, the assistant attorney general for the Office of Legal Counsel.

That claim was rejected by Senator Tim Kaine, Democrat of Virginia, who has regularly argued that Congress is failing to live up to its constitutional role in making decisions about war and peace. He called the argument that firing missiles at a foreign nation was not “war” nonsense.