U.S. Navy sailors staff the Arleigh Burke-class guided-missile destroyer USS Donald Cook as the ship departs Larnaca, Cyprus, April 9. (Alyssa Weeks/U.S. Navy/Reuters)

The Syrian government’s use of chemical weapons on its own citizens has attracted wide condemnation — and a tweeted heads-up from President Trump to Syria’s chief ally about America’s “nice and new and ‘smart!’ ” missiles.

“Get ready Russia, because they will be coming,” Trump warned. Syrian leader Bashar Assad even earned a new nickname: “Gas Killing Animal.”

That is one of the president’s more accurate Twitter insults. But as I wrote almost exactly a year ago in this space,

U.S. intervention in the Syrian civil war might, or might not, be the right thing to do. Bombing the Assad regime is certainly viscerally satisfying. But without legislative authorization, it is hard to think it’s legal.

That post stemmed from Trump’s airstrikes on a Syrian air base after an earlier chemical attack on civilians — which obviously did not deter the Assad regime from repeating such attacks. Since then, U.S. troops have fought — and died — in an increasingly wide geographic swath of the Middle East and North Africa. All this has been justified legally by pointing to the Authorization for the Use of Military Force passed in September 2001, shortly after the 9/11 attacks.

Building on a legal rationale developed by the Obama administration that the 2001 authorization applied to the fight against the Islamic State — as an “associated force” of al-Qaeda — Trump administration officials claimed in January that it could be used against Syrian troops “to defend U.S., coalition, and partner forces engaged in the campaign to defeat ISIS to the extent such force is a necessary and appropriate measure in support of the D-ISIS campaign.”

This is already problematic: Syria is a sovereign nation that had nothing to do with the 9/11 attacks, and the 2001 authorization was written to respond to whomever “planned, authorized, committed, or aided” those attacks. Is the Syrian military now an associated force of an associated force? Over how many degrees of separation does the authorization stretch?

Further, the associated force rationale does not obviously extend to offensive airstrikes against Syria unrelated to defending U.S. or coalition troops. Indeed, in the letter just cited, the Pentagon noted that the April 6, 2017, strikes were not authorized on the 2001 authorization: “the President authorized that strike pursuant to his power under Article II of the Constitution as Commander in Chief and Chief Executive to use this sort of military force overseas to defend important U.S. national interests.”

There is one large problem with this rationale: Article I of the Constitution, which gives Congress, not the president, the power to declare war. The title of commander in chief does not supersede that responsibility.

To be sure, past presidents have often used force without seeking prior congressional authority. But those circumstances generally had common characteristics not present in the Syrian case — something President Barack Obama’s lawyers ran up against as they tried to craft a rationale for striking that government after yet another chemical atrocity in 2013.

Back then, Donald Trump strongly felt that Obama should seek congressional approval. And the former head of George W. Bush’s Office of Legal Counsel, Jack Goldsmith, wrote that “an intervention in Syria would extend the president’s war powers under the Constitution beyond where they have gone before.” This is because in the past, when presidents have used force without legislative authority, they have been able to claim some measure of self-defense or international mandate.

In the first category might fall Jimmy Carter’s (failed) 1980 rescue attempt of American hostages in Iran, or Bill Clinton’s 1998 missile strikes after al-Qaeda bombed U.S. embassies in Africa, or even Ronald Reagan’s 1986 bombing of Libya after an attack in Germany that killed a U.S. serviceman. More imaginatively, George H.W. Bush explained his 1989 invasion of Panama as a response to Gen. Manuel Antonio Noriega’s “reckless threats and attacks upon Americans in Panama [that] created an imminent danger to the 35,000 American citizens” there. Reagan justified his 1983 invasion of Grenada along similar lines: “American lives are at stake.”

Alternately — or sometimes in conjunction — presidents have relied on multilateral support or treaty demands. Reagan, in invading Grenada, was careful to stress that the Organization of Eastern Caribbean States had invited the United States to respond — and that it was doing so in concert with other nations in the region, albeit nations with rather fewer military assets. As he said, “this collective action has been forced on us by events.”

Similarly, in entering Somalia (1992) and attacking Kosovo (1999) and Libya (2011), U.S. administrations were able to cite both humanitarian concerns and treaty obligations with the United Nations, NATO or both.

All these attacks certainly frayed War Powers Resolution limits. For instance, the WPR specifically rules out inferring authority to use force from treaty obligations. Nonetheless, such circumstances gave the presidents cover, and Congress failed to overturn their actions.

Since last April, Congress has shown intermittent interest in considering a new Authorization of the Use of Military Force that would place the U.S. fight against the Islamic State on a firmer legal footing. The potential expansion of American actions in Syria makes a new authorization — or perhaps separate authorizations, for the Islamic State and Syria proper — all the more urgent.

As I wrote a year ago, “The Syrian civil war certainly presents a tragic humanitarian dimension and near-constant violations of international norms. But these are not new this week; nor, as yet, is there an international mandate for intervention there. If there is to be a domestic mandate, it must come from congressional deliberation.”

A year later, there is plenty of bluster from the White House corner of the Internet. But as yet, from Capitol Hill … crickets.