This has been updated with the latest news.

Constitutionally, only Congress can declare war, so the president needs Congress’s approval for sustained military conflict. That’s the primary contour of the 1973 War Powers Act, which was passed in the wake of the Vietnam War to prevent another drawn-out, undeclared war.

On Wednesday, House Speaker Nancy Pelosi (D-Calif.) announced the House will use its authority under this law to try to curb President Trump’s actions in Iran. It’s not immediately clear what that looks like, but she said a vote will happen Thursday.

So what is the War Powers Act and how can Congress use it to stop a president’s military action they don’t approve of? Here’s an explainer.

First, what are war powers?

Letting Congress declare war sounds simple enough, but it’s actually tricky, mostly because: What’s war? Is it launching Tomahawk missiles at a Syrian chemical weapons program, as Trump has done in the past? Is it taking out a senior government official of a hostile country, as Trump did with Iranian commander Qasem Soleimani?

Recognizing war powers is kind of a know-it-when-you-see-it thing, experts have told The Fix as we’ve analyzed this over the years. But Congress has been reluctant to jump in and assert its authority to declare war because doing so carries such a high political risk.

That’s largely how presidents since George W. Bush have escaped congressional scrutiny for military action with or in Iraq, Afghanistan, Syria and now, Iran. Here’s a look at what a president can do militarily without Congress, what Congress can do to force the president’s hand, and why it’s an uphill battle in Congress to do that.

What military action can a president can take without Congress?

A president’s unilateral power is “something short of war. It’s the use of force by the president to achieve an immediate end,” said Phillip Carter, a senior fellow with the national-security-focused Center for a New American Security think tank, speaking to The Fix shortly after Trump launched a missile strike on Syria in April 2017 to retaliate for that regime’s use of chemical weapons.

One key and somewhat obvious indicator that Congress should be notified within 48 hours of sending troops into conflict, as the War Powers Act indicates, is if the president does something that risks triggering a sustained military conflict, said Cornell Law Dean Jens David Ohlin in an email to The Fix. That appears to be the case here: Iran has promised to retaliate, and the Pentagon has told thousands of troops to be ready for action in the Middle East.

“Legal scholars have long recognized that the need for congressional approval increases with the possibility of escalation, even in the case of an isolated military operation,” Ohlin wrote. “For this reason, Trump should have consulted Congress, and arguably should have sought approval for, a military action that could escalate to all-out war with Iran.”

But Trump didn’t notify Congress, and now he’s claiming this tweet should count as enough notification for anything and everything he does hereafter:

Experts The Fix talked to after Trump launched a missile into Syria in 2017 said that strike was probably okay without congressional approval, as long as it was a one-off. Carter also pointed out that President Ronald Reagan didn’t seek congressional approval when he bombed Libya in retaliation for a bombing of a Berlin club in 1986. In the ’90s, both President George H.W. Bush and President Bill Clinton ordered bombings in Iraq on their own, in between the Persian Gulf War and the Iraq War.

What Congress can do to reassert itself?

That’s a tough question, because experts largely agree that lawmakers over the years have let their jurisdiction over declaring war slip over to the executive branch.

To take it back, they would probably have to make a bold move. Passing a resolution like the House might to tell Trump he can’t launch missiles in Iran, for example, is a bold move.

Pelosi also says the House is considering repealing the legal authorization Trump has used thus far to carry out attacks in Iran and Syria, and to defund anything related to Iran. (Since Congress’s other main function is power of the purse.)

But these resolutions don’t initially seem to have any Republican support and thus less of a chance to make it to Trump’s desk and affect his Iran policy.

Why it’s been so difficult for Congress to regain its control over war powers

Congress has spent the past two decades struggling to come to a consensus what kind of military action to authorize for presidents. It’s a really hard thing to agree to, policy-wise and politically.

The only two authorizations of military force on the books right now are nearly 18 years old and arguably out of context with everything we’re talking about here. In the years after the Sept. 11, 2001, terrorist attacks, Congress authorized President George W. Bush to battle al-Qaeda.

President Barack Obama recognized this limitation and tried to get Congress to pass a new authorization of military force to allow him to go after the Islamic State, even as he somewhat awkwardly argued he had enough authority under the Bush AUMF. (AUMF is what these things are called in D.C. speak.)

But Obama never got that vote. His request for a three-year authorization of force in Syria got torn apart between liberal Democrats and libertarian Republicans who didn’t want to okay military action in any form, and hawkish Republicans who wanted to give the president more authority than he asked for.

Those factions still exist in Congress today. On the whole, Democrats are skeptical of giving Trump more authority to wage conflict abroad, while Republicans seem okay letting the president take unilateral action. In fact, a number of Republicans commenting about Trump’s Soleimani killing don’t see a need for a new AUMF.

But even if there were a bipartisan consensus that Congress needed to put guardrails around Trump, it is very difficult to get lawmakers to take such a difficult vote, especially in an election year.

They would be on the record authorizing an unknown scale of conflict, for an unknown amount of time. (You can put limitations on AUMFs, but the recent past has shown presidents blow right past those.) Remember when Hillary Clinton’s 2001 vote to authorize the Iraq War helped Obama win the 2008 Democratic primary?

AUMFs also don’t end with administrations. So lawmakers will also be on the record authorizing conflict for presidents other than Trump, as soon as next January. Why would Republicans in particular authorize such expansive powers for a potential Democratic president?

We haven’t even gotten into the nitty-gritty details of how long such an authorization should be for, what countries it should encompass, and what kind of military action it should encompass.

Wrap it all up, and you can see how difficult it is for Congress to do anything to put limitations on Trump as the potential for another conflict in the Middle East escalates, and to enforce its right to know about sustained military conflict before it happens.