This post, originally published in April 2017, has been updated with the latest information. 

To hear some members of Congress tell it, President Trump's strike on the Syrian government's chemical weapons program early Saturday was illegal. Congress, after all, is the only branch of government that can authorize war.

But foreign policy experts The Fix spoke to after Trump's first Syria strike a year ago say Trump is probably okay launching one or even a couple 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. As Sen. Tim Kaine (D-Va.) points out above, 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, speaking to The Fix shortly after Trump launched the first missile strike on Syria in April 2017.

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 Syrian 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.

The Post's Anton Troianovski and Louisa Loveluck explain why the joint United States military strike against Syria on April 13 will likely have little effect. (Anton Troianovski, Louisa Loveluck, Joyce Lee/The Washington Post)

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 that Trump expressed a desire to bomb. And those authorizations don't come close to addressing the target of Trump's 2017 missile strike: 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.

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.

But it's unclear that GOP hawks alone — who applauded Trump's first military strike last April  — 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 a concrete strategy on Syria. Earlier this month, he instructed the military to begin to withdraw from the country, to the strong criticism of some Republican senators.

“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 States 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 pass constitutional muster. From there, it's an open question what he can legally do, and, once he decides, whether Congress will let him.

U.S. coalition strikes Syria with cruise missiles

The guided-missile cruiser USS Monterey fires a Tomahawk land-attack missile at Syria as part of an allied strike. President Trump ordered a joint force strike on Syria with Britain and France in response to a recent suspected chemical attack by Syrian President Bashar al-Assad. (U.S. Navy/Getty Images)