President Donald Trump late Sunday informed Congress in writing of his decision to order a U.S. missile strike against Syria.

Under the War Powers Resolution, the president must keep Congress informed of such actions.

Trump's letter to congressional leaders cited the rationale he gave publicly Friday night when he announced that the U.S. and allies Britain and France were firing missiles into Syria in response to an alleged poison gas attack on the moderate opposition near Damascus the previous week. He wrote that the targets were Assad regime military chemical weapons-related facilities.

The president also told lawmakers that he acted to "promote the stability of the region, to deter the use and proliferation of chemical weapons, and to avert a worsening of the region's current humanitarian catastrophe."

The U.S.-led operation won broad Western support. The NATO alliance gave its full backing; NATO Secretary-General Jens Stoltenberg said in Brussels that the attack was about ensuring that chemical weapons cannot be used with impunity.

British Prime Minister Theresa May said in London that the West had tried "every possible" diplomatic means to stop Assad from using chemical weapons. "But our efforts have been repeatedly thwarted" by Syria and Russia, she said.