President Trump’s decision to bomb a Syrian airfield in response to a deadly chemical weapons attack has inspired a more bipartisan response from Congress than anything else the president has done in his 11 weeks in office.

From House Minority Leader Nancy Pelosi (D-CA) to former Democratic vice presidential nominee Sen. Tim Kaine (D-VA), from Sen. Rand Paul (R-KY) to Senate Armed Services Committee Chair John McCain (R-AZ), huge swaths of lawmakers from both parties agree: What Syria’s Bashar al-Assad did was terrible, but if Trump wants to commit the US military to doing more to fight him, he ought to go to Congress for approval.

He ought to get, they argue, a new Authorization for Use of Military Force (AUMF): formal endorsement from Congress of the US’s involvement in military “hostilities” against a particular enemy or set of enemies.

It remains to be seen, of course, whether the Trump administration will actually listen — the administration hasn’t clearly said whether it plans to take broader military action against Assad, let alone whether it will seek congressional authorization if it does. And if the Trump administration does try to go it alone, it’s unclear whether Congress will care enough to assert its authority over the executive branch.

The past 20 years — stretching back even before the “war on terror” began in 2001 — suggest it won’t.

It would be perfectly normal for the Trump administration to argue it doesn’t need Congress’s approval to go after Assad. It would court a fight over when and why the executive branch can get America’s military involved in operations overseas without input from Congress — but that fight has been going on for decades. Indeed, the Obama administration made legal arguments for justifiable unilateral force that could be interpreted to validate what Trump did on Thursday night.

If Congress really wanted to, it could stop the slow creep of executive war power by forcing Trump to ask, explicitly, for approval to go after Assad. Or it could issue a strong endorsement of an intervention that could turn out to be politically popular.

Both of those are things an AUMF can do. But it can’t do both of them at once. Getting Congress to agree on an AUMF will require the members who want to expand presidential war powers, and the members who want to curtail them — a split that doesn’t break evenly along party lines — to come together on a single plan and then put their names to it with a vote.

It’s often easier, Congress has found, to just let the executive do what it wants.

Why presidents need Congress to authorize military force — at least in theory

”It is a remarkable fact about the US Constitution,” Harvard Law School professor Jack Goldsmith wrote Friday morning at the blog Lawfare, “that 228 years after its creation, we still don’t know what limits, if any, it imposes on unilateral presidential uses of military force.”

Article II of the Constitution says that the president is the commander in chief of the armed forces — which means he’s responsible for directing them into battle. But Article I of the Constitution gives Congress, and Congress alone, the authority to declare war — and to appropriate funds to the Defense Department to wage it.

There’s no bright line about when something counts as a war that Congress must approve, and when it’s simply a military action the president can direct. (All of this is separate from the question of whether a given military operation is legal under international law — which is also a valid question about Trump’s strikes against Assad.)

For most of American history, this wasn’t a problem. ”From 1789 to 1950, Presidents came to Congress either for a declaration of war or statutory authority whenever they decided it was necessary to take the country from a state of peace to a state of war,” Constitution Project scholar Louis Fisher told Ryan Goodman of the law blog Just Security on Thursday night.

“In doing so, they complied with the Framers’ clear intent that the decision to use offensive force against another country must reside solely in Congress,” he said.

President Harry Truman broke that precedent. While we call the Korean War a “war” today, Truman never bothered to get a declaration of war from Congress — he just sent US troops to aid American-aligned South Korea against communist North Korea on his own. That set a Cold War precedent. The Johnson administration got congressional approval (via the 1964 Gulf of Tonkin resolution) to escalate the war in Vietnam; the resolution was repealed in 1971, at which point the war it had written a “blank check” for had killed tens of thousands of American soldiers.

By the time Congress discovered that Nixon had conducted a secret bombing campaign in Cambodia, they got sufficiently fed up.

In 1973, Congress passed the War Powers Act, an attempt to clarify where the executive’s “commander in chief” powers ended and Congress’ “declare war” power began. The War Powers Act required the president to notify Congress when he got the US involved in “hostilities” — and then set a 60- to 90-day clock for Congress to approve that action, by passing an authorization of use of military force, or for the president to withdraw from the conflict. It also allowed Congress to pass a concurrent resolution that would force the executive branch to withdraw from any conflict it hasn’t already approved.

But the War Powers Act didn’t resolve the debate. It just shifted it — from what counted as a “war” to what counted as “hostilities.”

Some hawks (including some presidential administrations) have argued that the War Powers Act is itself unconstitutional, because being commander in chief gives the president unlimited authority to carry out military action. More often, though, presidents have just skirted the issue.

The only president to officially file a report that triggered the 60- to 90-day clock, according to the Library of Congress, was Gerald Ford. President Reagan went into Lebanon while telling Congress but without formally reporting on it, and into El Salvador without notifying Congress at all; President Bush argued that he didn’t need congressional approval to go into Kuwait, because the US was acting in support of a United Nations resolution. President Clinton told Congress when he went into Bosnia and Kosovo but never triggered the 60-day countdown — and while some members of Congress were upset by the move, it wasn’t enough to pass a resolution forcing the president to withdraw.

Then 9/11 happened — changing the debate from “when can a president take action without authorization?” to “just how far does one authorization stretch?”

2001: one AUMF, more than three dozen military actions

On September 18, 2001 — in the post-9/11 rush to “do something” that also brought America the Patriot Act and the Department of Homeland Security — Congress passed an authorization of military force that allowed the president to:

use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons.

The 2001 AUMF justified the war in Afghanistan, since al-Qaeda was the organization that had planned the 9/11 attacks and the Afghan Taliban had “harbored” al-Qaeda. (Congress passed a separate AUMF, in 2002, to authorize the war to oust Saddam Hussein in Iraq.) But it opened the door to other conflicts as well — and didn’t set a time limit for how long the authorization would stand.

So the executive branch has just kept using it. By May 2016, according to a Congressional Research Service report, “the current authorization [had] been used to justify unclassified military action 37 times in 14 countries since 2001” — 18 times by President George W. Bush, and 19 times by President Barack Obama.

Some of these interventions were limited — like a 2004 effort in Georgia — while others, like efforts to combat terrorists in Somalia and neighboring countries, have stretched on for years on end.

To many members of Congress, in both parties, that’s started to seem a little ridiculous.

For one thing, the AUMF is now 15 years old. But more importantly, the AUMF didn’t authorize action against any terrorist groups — it authorized action against the group (al-Qaeda) that attacked the US in 2001, and its affiliates. And the war on terror is no longer primarily a war on al-Qaeda, and hasn’t been for a long time. Al-Qaeda has lost power and prestige; other groups, like ISIS (which has its roots in al-Qaeda but is distinctly its own group), have gained it.

When President Obama launched a first round of US strikes against ISIS in 2014, experts were notably skeptical that a 2001 authorization could justify action against a group that didn’t exist in 2001: “it is an application of the AUMF that the legislature did not contemplate and could not have actually envisioned in September 2001, because Congress naturally did not foresee any such amoeba-like splitting of the enemy,” Georgetown law professor Marty Lederman wrote at the time.

Some members of Congress in both parties had been interested in limiting the scope of the 2001 AUMF for a while. In 2013, a proposal to stop funding for any AUMF-authorized activities after 2014 (led by Rep. Adam Schiff (D-CA), now the ranking member on the House Intelligence Committee) failed in the Republican House. Many Democrats supported efforts to limit the AUMF, as did libertarian Republicans like Rand Paul and anti-Obama conservatives like Rep. Louie Gohmert (R-TX); on the other side of the aisle, some moderate Democrats resisted the idea of limiting Obama’s authority.

Then Obama did something so unusual that congressional Democrats didn’t understand it (and were kind of mad about it) at the time: He asked Congress, in February 2015, to pass a new AUMF specifically to authorize the fight against ISIS, even though he argued he didn’t actually need one. In other words, he asked Congress to limit the power he had already been given.

As Vox’s Matt Yglesias explained at the time, this was in part because Obama anticipated the US would have a more hawkish president in 2017 (though he surely didn’t anticipate it would be Donald Trump), and wanted to make sure that president had clearer authority about when he or she could send US troops and bombs abroad.

But in 2015, Congress had a hard time agreeing on anything, and certainly an authorization to use force the president was already employing. So the 2001 AUMF still stands.

But even the 2001 AUMF can only stretch so far.

Trump didn’t go after Bashar al-Assad because his government was harboring terrorist groups that were responsible for 9/11. And no one is arguing that he is. (In fact, Assad claims he’s prosecuting a civil war in Syria to fight terrorist groups like ISIS.)

“The constitutional legality of last night’s strike against Syrian military forces in response to chemical weapon attack earlier this week,” Harvard Law’s Goldsmith wrote on Friday morning, “can only be justified by Article II” — by the commander in chief’s inherent powers — not by an existing congressional authority.

In other words, Trump doesn’t need to use the 2001 AUMF. He can justify what he did on Thursday night by arguing he doesn’t need any AUMF at all.

Why Obama thought he wouldn’t need an AUMF to go after Assad for chemical weapons

After all, a presidential administration already decided that it could take action against the Assad regime for using chemical weapons, without needing Congress to weigh in. That was in 2013, and the administration was Barack Obama’s.

When presidents have sidestepped the War Powers Act in the past, they haven’t (usually) just asserted that they can do whatever they want. Instead, they’ve laid out specific standards for when something counts as “hostilities” that merit congressional approval — and then argued that whatever they’re doing at the time doesn’t meet that standard. (Georgetown law professor Lederman has a post at Just Security that goes into more depth about the various arguments presidents have used to justify unilateral action.)

As those arguments have built on each other from president to president, they’ve erected something like a consistent framework. Basically, presidents before Trump argued they didn’t need authorization if:

What they were doing was obviously in the US’s “national interest,” and The military action was strictly limited in time and scope (in other words, it wasn’t big enough to count as a real war, or even real hostilities)

That two-pronged test is how the US justified involvement in the (technically, but only nominally, NATO-led) war in Libya in 2011, another fight against a sovereign nation that wasn’t justified on terrorism grounds. (There’s substantial disagreement about whether the Obama administration was right to go into Libya without an AUMF, and the procedure by which Obama decided the Libya war was legal was a little suspect.)

And in 2013, when it was discovered that Bashar al-Assad had used chemical weapons against Syrian civilians in violation of international law — crossing President Obama’s famous “red line” — the administration weighed whether that test applied to a potential strike against Assad as well.

It decided that it did: that protecting an international norm against the use of chemical weapons, among other things, was an important national interest. But it decided, just like it later would in 2015, to ask Congress for approval anyway — because the administration knew it would be setting a precedent by not getting an AUMF, and that was a precedent it didn’t want to set. As Clinton administration official Walter Dellinger told the New York Times at the time, “[W]hen the president is going beyond where any previous president has gone, it seems appropriate to determine whether Congress concurs.”

The whole thing turned out to be moot when Russia persuaded Assad to give up his chemical weapons in exchange for the US backing away from using military force (something it’s now tragically clear that Assad didn’t fully do). Now, in 2017, President Trump has decided to do what President Obama did not.

The Trump administration hasn’t put out any explicit legal justification for the strike, or even clarified whether it plans to seek an authorization of military force against Assad.

Still, you can already see Trump laying the groundwork for an argument that Thursday night’s strikes satisfied Obama’s two-part test — and therefore no AUMF is needed. His statement Thursday night cited norms against chemical weapons, and the Syrian civil war’s role in shaping the global refugee crisis and regional instability, as problems for the US “national interest.” He also stressed that Thursday’s strikes were limited and surgical.

But while the Obama administration thought that argument might be valid against Assad, they didn’t want to unilaterally assert that it was — and so they wanted to seek congressional approval anyway. To do otherwise, Lederman wrote at the time, would be “an unprecedented basis for unilateral executive action, and it would open up a whole new category of uses of force that Presidents might order without congressional approval, even where such actions could have profound, longstanding consequences.”

If Trump decides to use Obama’s rationale to act unilaterally where Obama didn’t, that might be the door he’d be opening.

Even if Congress says it wants to do something, it can’t agree on what should be done

The fact that powerful lawmakers from both parties are saying they want a new AUMF is noteworthy — after all, even before the strikes against Assad, there was some interest in updating the 2001 AUMF (something Defense Secretary James Mattis appears to support). But it doesn’t mean anything will actually happen.

This isn’t just because Congress has trouble getting anything done these days. It’s because the reasons some members of Congress want a new AUMF are diametrically opposed to each other.

For one thing, many Republican hawks don’t believe a new AUMF is needed — or that the War Powers Act is even constitutional. Senate Majority Leader Mitch McConnell (R-KY) said Friday that he’d be interested in looking over an AUMF on Syria “if the president feels he needs one” — implying that the decision about whether the executive branch had the power to get further involved in Syria was entirely up to the executive branch.

McConnell’s lackadaisical attitude reflects what happened in 2011, when President Obama went into Libya without asking for congressional approval. Just before the 90-day window — the maximum for congressional authorization — was set to expire, House Speaker John Boehner sent the Obama administration a letter asking if they felt the Libya war didn’t need an AUMF, or if the Obama administration’s lack of notification meant they thought the War Powers Act was unconstitutional (an opinion Boehner himself held).

The administration replied the next week, finally explaining its reasoning for why the Libya war didn’t need an AUMF (the two-pronged test of “national interest” and limited scope).

The Senate Foreign Relations Committee continued to question the need for an AUMF in Libya, and called in State Department official Harold Koh to defend the administration’s reasoning. But ultimately, there wasn’t enough opposition to Obama’s actions in Libya to take congressional action. (A handful of members of Congress tried to sue the Obama administration for violating the War Powers Act — something that also got tried when President Clinton went into Kosovo — but both attempts were thrown out of court.)

It’s possible that members of Congress might be more inclined to push back against Trump in 2017 than they were against Obama in 2011. But “do we need an AUMF” is only the first question. The second question is what the AUMF ought to do.

Some members of Congress, mostly Democrats — but also Republicans like Rand Paul — want an AUMF so that there are clear limits on the president’s authority in any given conflict. Many of them opposed President Obama’s attempt to pass an ISIS AUMF in 2015 because it didn’t fully rescind the old, stretched-out 2001 AUMF.

But some Republicans want a new AUMF to make it clear that the president has very broad powers; they opposed the 2015 AUMF because they didn’t like Obama’s attempts to limit the power of his office, and they’re hardly more likely to support limits on presidential power now that the president is a Republican.

This is the dynamic that’s kept the efforts to reform the 2001 AUMF in limbo. And it applies just as much to the question of a new AUMF against Assad. Is an AUMF needed so that the president, legally speaking, has the tools he needs? Or is it needed so that he isn’t allowed to use all the military tools he has?

Because many members of Congress believe the president doesn’t need a new AUMF, they have little incentive to agree to pass one that they think would limit his powers. And even members who don’t feel strongly on the issue would have to be talked into an on-the-record endorsement of military action that could potentially erupt into full-blown war.

That prospect isn’t appealing when the alternative is to just allow President Trump to do what he wants in Syria, using the most aggressive interpretations of already aggressive arguments made by his predecessors to set a new standard for unilateral actions that are war in all but name.