Washington Post reporter Dan Lamothe explains why President Trump launched 59 Tomahawk missiles at a Syrian military airfield on April 6 and what this means for the fight against the Islamic State. (Sarah Parnass,Julio Negron/The Washington Post)

To hear some members of Congress tell it, President Trump's sudden and direct attack on the Syrian government Thursday night was illegal. Congress, after all, is the only branch of government that can authorize war.

But foreign policy experts The Fix spoke to say Trump is probably okay launching one or even a couple series of strikes on his own without Congress's permission. That's because the War Powers Act allows the president to take some military action on his own, as long as it's more of a one-off thing and not long term.

If Trump wants to go any further — well, then our experts say he's going to need Congress's blessing. Trump himself thought the same thing when Obama was considering military strikes against Syria in 2013.

The line on when a president needs Congress's approval is fuzzy, but it's more of a know-it-when-you-see-it-situation, said Phillip Carter, a senior fellow with the national security-focused Center for a New American Security think tank.

A president's unilateral power, he said, is "something short of war. It's the use of force by the president to achieve an immediate end," he said. 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 dropped bombs in Iraq on their own, in between the Persian Gulf War and the Iraq War.

Jens David Ohlin, an associate dean at Cornell Law School, agreed that Trump is in the clear for now: "He would need congressional authorization if he wants this to be a sustained attack, but if this is just a one-off attack, then I don't think he really needs congressional authorization."

But even if Trump had not ordered the United States' first direct military strike on the Syria's government since that country's civil war began six years ago, he would probably need to go to Congress to get authorization just to keep up the status quo from the previous administration.

President Barack Obama never launched Tomahawk missiles at a Syrian government airfield, but even his drone strikes in the region were legally flimsy.

That's because the only two authorizations of military force on the books right now are 15 years old and arguably out of context with the actions of both Trump and Obama. In the year after the Sept. 11, 2001, terrorist attacks, Congress authorized President George W. Bush to battle al-Qaeda.

Most scholars would agree that is a separate terrorist group from the Islamic State that Obama was targeting and Trump has expressed a desire to bomb. And those authorizations don't come close to addressing the target of Trump's missile strike Thursday: the airfield of a nation's military.

"There are few more slippery slopes than that to war," Carter said. 

Obama never launched Tomahawk missiles at Syria's government, but for much of his presidency, he tried to square the circle of a 15-year-old authorization and his desire to get involved in Syria: He kept asking Congress to authorize him to use military force in Iraq and Syria against the Islamic State, even as he maintained that the authorizations already in place were valid.

In this image provided by the U.S. Navy, the guided-missile destroyer USS Porter (DDG 78) launches a Tomahawk cruise missile in the Mediterranean Sea, Friday, April 7, 2017. The United States blasted a Syrian air base with a barrage of cruise missiles in retaliation for this week's chemical weapons attack against civilians. (Mass Communication Specialist 3rd Class Ford Williams/U.S. Navy via AP)

Obama never managed to get that approval from Congress. His request for a three-year authorization of force in Syria got torn apart between liberal Democrats/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. (In 2013, more than 100 mostly Republican House lawmakers signed onto a letter demanding that Obama seek congressional approval for what Trump just did.)

There were also a number of Democrats and Republicans who were wary of authorizing the White House's war powers in an election year, when they weren't sure who else they would be handing the keys to the bombers.

Many of those dynamics may have changed now that Trump is president. Yes, Trump has a hands-off worldview on global affairs, but he's also been supportive of expanding the powers of the presidency. Ohlin thinks that if Trump went to a Republican Congress and asked for a broad authorization of military force, it would be willing to give it to him.

Lawmakers from both political parties weigh in on President Trump's decision to order airstrikes in Syria on April 6, in retaliation for a chemical attack on civilians. Many senators lauded the attacks as "proportionate," but split on what Trump's next steps should be. (Bastien Inzaurralde,Jenny Starrs/The Washington Post)

But it's unclear that GOP hawks alone — who applauded Trump's strike Thursday — could pass a broad authorization of force for the president. There are plenty members of Congress on both sides who first want to know what Trump plans to do with it. So far, he has not articulated any kind of strategy. (His administration is in the midst of a large-scale review of what it wants to do in Iraq and Syria.) But what he has articulated has been erratic: Just days earlier, he was coming under fire from GOP hawks for wanting to let Syrian President Bashar al-Assad stay in power.

"I don't think people know exactly where Trump is going to go with his foreign and military policy," Ohlin said, "and he may not even know."

The tug-and-pull between Congress and a president on the authorization of military force is nothing new. It's a messy, ambiguous thing, and smart people on all sides can credibly make the case for a line to be drawn pretty much anywhere: There are people who argue that it's unconstitutional for Congress to even have the power to authorize war. And still others argue it's illegal in international law for the United Statres to intervene in Syria.

The U.S. courts have stayed out of this, Ohlin said, preferring to let the other two branches duke it out.

But for our purposes, the consensus view is that the president's military strikes Thursday night pass constitutional muster. From there, it's an open question what he can legally do, and, once he decides, whether Congress will let him.

This report originally incorrectly stated which international bombing President Reagan's 1986 bombing of Libya was in retaliation for.