VIDEO: https://youtu.be/o_CPINFuHhI

WASHINGTON ' Today, in a hearing of the Senate Appropriations Subcommittee on Defense, U.S. Senator Tom Udall pressed Secretary of Defense James Mattis and Chairman of the Joint Chiefs of Staff General Joseph Dunford on the need for Congress to debate an Authorization for the Use of Military Force (AUMF) for the ongoing and growing U.S. military involvement in Syria.

As Udall said, the Constitution clearly requires Congress - not the president - to make declarations of war. However, in spite of public reports of up to 1,000 U.S. troops deploying to Syria, Congress still has neither debated nor passed an AUMF authorizing military action in Syria. Instead, the president is relying on a more than 15-year-old AUMF that was passed after the September 11th attacks.

"U.S. military vehicles and heavy artillery have been seen in Syria. It is easy to argue that the United States has effectively invaded northern Syria, violating the sovereignty of a country in the Middle East, which is a de facto declaration of war," Udall said.

"Are you concerned that Congress has not approved an AUMF specific to Syria, granting the President the legal power to invade Syria?" Udall asked Mattis and Dunford.

"I would take no issue with the Congress stepping forward with an AUMF," Mattis responded. "I think it would be a statement of the American people's resolve if you did so. I've thought the same thing for the last several years, I might add, and have not understood why the Congress has not come forward with this - at least the debate."

"I think not only would [an AUMF] be a sign of the American people's resolve, but truly, I think our men and women would benefit from an Authorization for the Use of Military Force that would let them know that the American people, in the form of their Congress, were fully supportive of what they were doing out there every day, as they put their lives in harm's way."

Udall, also a member of the Senate Foreign Relations Committee, has long led the call for Congress to debate and approve an AUMF to limit the cost and harm to U.S. troops and ensure the United States does not begin another indefinite conflict in the Middle East. He has opposed sending ground troops to Syria, and escalating U.S. involvement in the civil war. Along with Sens. Mike Lee (R-Utah) and Chris Murphy (D-Conn.), he introduced bipartisan legislation in 2013 and again in 2015 to prohibit the administration from using any funds on military aid to rebel groups and covert activities that would escalate U.S. involvement in the Syrian civil war.

Below is the full text of Udall's opening statement as prepared for delivery.

"Secretary Mattis and General Dunford, thank you for your time and your years of service to the nation.

"The president is asking Congress for a defense supplemental appropriation for fiscal year 2017 and a significant increase in defense spending in 2018.

"I want to discuss these requests in the context of our current war against ISIL in both Syria and Iraq.

"In the last couple of weeks, the American people have read public reports that additional Marines and other U.S. forces have deployed to Syria ' up to 1,000 troops.

"The American people are being told that they are there to help counter ISIL.

"To paraphrase St. Thomas Aquinas, one of the first and most important requirements to wage a just war is that the war must be ordered by a legitimate authority.

"Alexander Hamilton and other founders argued strenuously to ensure that this power was granted to the Congress, rather than the Executive, in order to prevent the president from engaging in risky overseas ventures.

"As you know, the responsibility to make decisions about war and peace is one of the most important powers given to the Congress under the Constitution.

"When the Iraq war first began, we were not an invited force, but an invasion force ' an invasion that required an authorization for use of military force from the Congress.

"Fourteen years later, we still have United States forces in Iraq serving in a train and assist role, and they have been invited by the Iraqi government to support its efforts to counter ISIL.

"And the 2003 invasion of Iraq required an AUMF specific to Iraq.

"But in Syria, on the other hand, the U.S. has not been invited by the government. U.S. military vehicles and heavy artillery have been seen in Syria. It is easy to argue that the United States has effectively invaded northern Syria, violating the sovereignty of a country in the Middle East, which is a de facto declaration of war."