President Trump’s recent decision to authorize missile strikes on April 7 against the Syrian regime in retaliation for a deadly chemical attack has provoked the debate over a new authorization for the use of military force (AUMF) to resurface in Congress.

The debate is far from clear-cut because there are lawmakers on both sides of the aisle who appear to believe Mr. Trump had the legal authority to take such action, while others think he should have sought congressional approval. Some hold murkier positions on the issue.

In the post-World War II era, lawmakers have typically approved authorizations for most military conflicts, including the Vietnam war, the Persian Gulf war, the Iraq war and the invasion into Afghanistan after the 9/11 terrorist attacks. The 2001 AUMF that authorized the U.S.-led invasion into Afghanistan contained broad language that provided leeway taken by President Obama in the war against terror.

In 2013, Obama wrestled with striking Syria after a deadly chemical attack outside of Damascus. Initially, he threatened a military strike against the Assad regime and suggested there was a legal basis for acting unilaterally, but then he shifted and sought congressional approval. Before a House and Senate vote could take place, though, Russia stepped in and helped facilitate an agreement seeking to rid Syria of its chemical weapons stockpile.

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Defense and foreign policy experts say the precedent that Obama set for unilateral action might be why the uproar among lawmakers after the latest strikes hasn’t been as pronounced as four years ago.

“I think there’s one other aspect to this -- what are we doing when we strike this way?” said Elliott Abrams, who served as deputy national security adviser under President George W. Bush. “We are enforcing international law, we are enforcing the chemical weapons convention. We’re acting, as Secretary Tillerson put it, ‘on behalf of the international community,’ and I think a lot of Democrats are surprised to see President Trump doing that and are happy to see him doing that.”

In the last few years of the Obama administration, the conversation on Capitol Hill over a new AUMF focused on U.S. military operations targeting the Islamic State in Iraq and Syria (ISIS). Without a congressional green light, the U.S. military began airstrikes over Iraq in August 2014 and expanded to strikes against ISIS in Syria two months later.

At first, Obama had said he had the proper authority to launch those strikes, but the White House later sent a proposed AUMF to Congress in February 2015. Lawmakers tried to tackle the issue, but internal divisions within both parties prevented them from reaching a consensus.

“If Congress believes, as I do, that we are at war with [ISIS], it should go ahead and vote to authorize the continued use of military force against these terrorists,” Obama said in a televised address to the nation in December 2015. “For over a year, I have ordered our military to take thousands of airstrikes against [ISIS] targets. I think it’s time for Congress to vote to demonstrate that the American people are united and committed to this fight.”

The failure to reach a consensus is because lawmakers are split into multiple camps. First, there seem to be those who believe that an update to the 2001 AUMF is not needed, while others think a new one is necessary because the 2001 version is outdated and irrelevant due to its focus on al Qaeda.

But within that latter camp, there are lawmakers from both parties who want to dramatically limit the scope of the president’s authority and by contrast, those who are more hawkish and want to ensure broad, expanded authority.

“I do think it’s a mistake that Congress has chosen to ignore for decades or well over a decade, some would say for many decades, the declaration of war power,” Sen. Mike Lee, R-Utah, said on CNN ahead of the missile strikes against Syria. “We need to pass a declaration of war or at least an explicit authorization for the use of military force.”

After the strikes, Rep. Karen Bass, D-California, said on MSNBC, “[Mr. Trump] also won’t tell us what he plans to do next if there would be another strike. To me, that’s exactly why it’s so important for us to have the AUMF -- the authorization of the use of military force. This needs to be discussed and debated in Congress because we just simply don’t know what he’s going to do.”

Senate Majority Leader Mitch McConnell, R-Kentucky, said at a press conference ahead of Congress’ two-week recess, “I’d be interested in taking a look at an AUMF if the president feels like he needs it.”

House Minority Leader Nancy Pelosi, D-California, unsuccessfully urged Speaker Paul Ryan, R-Wisconsin, to call the House back into session during the current recess, which ends Friday.

Article I of the Constitution grants the power to declare war to Congress, but there have been few instances in the nation’s 200-plus-year history in which the legislative branch has embraced that authority.

“Of all the wars and all of the military actions that presidents have undertaken, that our country has been engaged in, there have been only 11 total declarations of war,” said Barbara Perry, director of presidential studies at the University of Virginia’s Miller Center.

Those wars include the War of 1812, the Mexican-American War, the Spanish-American War and then declarations against specific countries in World War I and World War II.

“Most of the wars in contemporary times have been fought without declarations and that doesn’t even get to...the kind of military action that the president took last week,” Perry added.

After the Syria missile strikes, 55 lawmakers sent a bipartisan letter to Mr. Trump saying that they’ve heard reports about the administration actively considering direct support for the anti-Houthi coalition of militaries led by Saudi Arabia in Yemen. They said that the 2001 AUMF would not justify taking that action.

“Engaging our military against Yemen’s Houthis when no direct threat to the United States exists and without prior congressional authorization would violate the separation of powers clearly delineated in the Constitution,” they wrote, adding that they request the Office of Legal Counsel to provide legal justification that the administration would cite if the U.S. decides to engage in direct hostilities against Yemen’s Houthis.

Given the missile strikes against the Syrian regime, this concern over Yemen and possible U.S. military action against North Korea has made the AUMF debate even more complicated than just crafting one against ISIS.

To break through the stalemate in Congress, an updated AUMF “will be driven by circumstances,” said David Adams, chief legislative adviser to Hillary Clinton when she served as secretary of State, who now serves as a principal at the Podesta Group.

The president, for example, could potentially launch additional strikes in Syria and put more U.S. troops on the ground there, said Adams, who added that that has also been an uptick in drone strikes against al Qaeda in the Arabian Peninsula (AQAP) in Yemen.

“I think what members will be looking at is where is the president going on both of those conflicts and is the temple of operations in both instances such that the members feel like they need to step in and get a handle on what’s going on here,” he said. “I think that, more than anything else, will drive the conversation up there in terms of getting to ‘yes’ on an AUMF.”

Abrams said he doesn’t know how likely a new authorization is, but worries that it’s much harder to achieve now.

“There was a good deal of talk about updating the AUMF in the last couple of years of the Obama administration. I’m sorry it wasn’t done then,” he said. “I think it would have been easier to do then and the partisan hostility was not quite as great then so I just don’t know now.”