Hours after American warplanes launched new strikes against pro-government Syrian forces, U.S. Sen. Tim Kaine called for the release of a secret Trump Administration memo, justifying unilateral use of force within the nation torn by civil war and the Islamic State.

The request is directed towards Secretary of State Rex Tillerson, after the memo was crafted and presented to the White House by the State Department. It remains unclear how much of the memo remains classified, but it has never been released to Congress or the American public.

“The fact that there is a lengthy memo with a more detailed legal justification that has not been shared with Congress, or the American public, is unacceptable,” Kaine wrote.

“I am also concerned that this legal justification may now become precedent for additional executive unilateral military action, including this week’s U.S. airstrikes in Syria against pro-Assad forces or even an extremely risky ‘bloody nose’ strike against North Korea.”

State Department personnel originally wrote the memo to justify President Trump’s April 6, 2017 missile strike from the Mediterranean – responding to President Bashar al-Assad’s use of chemical weapons against children and other civilians in Idlib Province.

American airpower again hit pro-Syrian military targets overnight Thursday, after U.S.-backed forces came under attack. Russia and Syria immediately condemned the U.S. strike, calling the American presence in Syria, “illegal.”

Under a new Syria strategy announced last month, American troops will remain in Syria until an elusive peace settlement is put in place.

The strategy also calls for U.S. troops to support the forces controlling roughly a fourth of Syria, until the transition of power begins from Bashar al-Assad begins.

With no end in sight to the conflict, concerns are growing that America may be drawn into another open-ended conflict in the Middle East, joining Afghanistan and a gradually dwindling force in Iraq.

“The Administration’s responses, if received at all, have been insufficient,” Kaine wrote. “This expansive view of the President’s Article II powers – absent an immediate threat to the United States or our personnel abroad – does not justify U.S. military action without the authorization of Congress.”

Article II designates the president as Commander in Chief of the Army and Navy of the United States, with the administration previously stating the president is able to use military force overseas to defend important U.S. national interests.

Kaine first sent a letter asking for more details on the U.S. justification of force in Syria in April 2017, then in December.

A copy of the letter can be viewed here.