Gregory Korte

USA TODAY

PALM BEACH, Fla. — President Trump did not rule out additional military action against Syria on Saturday, telling Congress that he was acting "in the vital national security and foreign policy interests of the United States" when he ordered the launch of cruise missiles at a Syrian air field this week.

Trump provided his justification in a letter to Congress under the War Powers Resolution, a 1973 law passed by Congress during the Vietnam War. That law requires the president to explain any new military action to Congress within 48 hours.

"I directed this action in order to degrade the Syrian military's ability to conduct further chemical weapons attacks and to dissuade the Syrian regime from using or proliferating chemical weapons, thereby promoting the stability of the region and averting a worsening of the region's current humanitarian catastrophe," Trump said in the letter.

The notification also starts a 60-day clock in which Congress must either authorize military force against Syria or the president must "terminate any use of United States Armed Forces" related to the conflict.

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Two U.S. destroyers in the Mediterranean Sea launched 59 Tomahawk missiles at the Shayrat military airfield in Syria early Friday. U.S. intelligence officials say they concluded that's the same air base Syria used to launch a poison gas attack against civilians in a rebel-held area of southern Idlib province.

Human rights groups said the airfield was once again operational within 24 hours, and that Syria was using it to launch new airstrikes.

On Twitter, Trump defended his tactical decision to use the cruise missiles to target infrastructure at the Syrian airfield, but not the runway itself. "The reason you don't generally hit runways is that they are easy and inexpensive to quickly fix (fill in and top)!" Trump tweeted from his resort at Mar-a-Lago, after spending more than five hours Saturday at the nearby Trump International Golf Club.

Trump said the United States will take additional action "as necessary and appropriate" to promote those interests.

"That is probably prudent," said Lou Fisher, a constitutional scholar and author of Presidential War Power. "He doesn't want Assad to think U.S. military power will be used only once."

The war powers notice — the first of Trump's presidency — mostly followed a template used by his predecessors.

Trump said he had the authority to order the strikes "pursuant to my constitutional authority to conduct foreign relations and as commander-in-chief and chief executive." And he told Congress he was providing them the notice "consistent with" the War Powers Resolution, language traditionally used by presidents to signal that they are complying with the law even while questioning whether Congress can encroach on the power of the commander-in-chief.

"Presidents have been doing these kinds of things for decades: Short, targeted military interventions or strikes, without congressional authorization," said William Howell, a presidential scholar at the University of Chicago and co-author of While Dangers Gather: Congressional Checks on Presidential War Powers.

"Invariably, there will be some members of Congress who will come out and say it’s unconstitutional, but it’s not clear what they can or will do about it as an institution," he said.