The Congress passed on Thursday US President Barack Obama's plan to train and arm moderate Syrian rebels to battle Islamic State jihadists, a key plank of his strategy to smash the militants.
The Senate voted 78 to 22 to authorise the first step in Obama's move to degrade and destroy the Islamic State group which has conquered swaths of territory in Iraq and Syria.
The measure now goes to the president's desk for his signature. Obama was scheduled to speak from the Oval Office at 2300 GMT.
The measure was attached to a must-pass, stop-gap spending bill that funds government only until December 11, meaning lawmakers will need to revisit the issue of US military authority in less than three months.
Lawmakers wary of triggering another West Asia war vowed to hold broad use-of-force debates later this year following the November 4 mid-term congressional elections.
"It's important that we give the moderates in the region a fighting chance," Senate Richard Shelby told the chamber.
But he also reflected the broad recognition that US air strikes — already under way in Iraq — and aiding Syrian rebels are merely first steps in what could be protracted US military action.
"There will come a time when more will be required to defeat this enemy. It will not be a short duration," Shelby said.
Just as House lawmakers said prior to their Wednesday vote to authorize the rebel training, some senators complained that Congress was ducking out of Washington this week without conducting a full debate on war authority.
"The Senate needs a real debate about our involvement in Iraq and Syria," Senator Jon Tester said.
"Are we putting another war in the Middle East on a credit card?"
Obama has pressed Congress to provide him political cover to initiate military action in Syria against IS, although the White House and many lawmakers believe he has the constitutional authority to launch air strikes in Syria, as he has done in Iraq, to protect US national security interests.
The Obama administration has insisted its military action is legal under the authorization of the use of military force (AUMF) that Congress passed in 2001 after the September 11 attacks.
In a provocative rant on the Senate floor warning of getting sucked into "perpetual war," Senator Rand Paul accused US officials and lawmakers of being "intellectually dishonest if you argue that something passed in 2001 to do with people who attacked us on 9/11 has anything to do with Syria."
The measure puts important checks on Obama's power, including a prohibition against US combat troops in Syria and requirements that the administration keep Congress in the loop with regular reporting about the operation.