Open this photo in gallery A Syrian soldier spray water on the wreckage of a building described as part of the Scientific Studies and Research Centre compound in the Barzeh district, north of Damascus, during a press tour organised by the Syrian information ministry, on April 14, 2018. LOUAI BESHARA/Getty Images

The United States is preparing to announce sanctions Monday against companies connected with Syria’s chemical-weapons program and push the United Nations for a full investigation of the Assad regime’s use of poison gas on the battlefield.

Coming three days after American, British and French air strikes against three of dictator Bashar al-Assad’s chemical facilities, the measures are meant to crank up the pressure on Syria to avoid a repeat of its attack on Douma earlier this month, during which the Syrian army is accused of deploying chlorine and sarin in retaking the city from rebel forces.

The moves – along with American warnings that Russia, Syria’s ally, has deployed internet trolls in a disinformation campaign about the air strikes – show an increased willingness by the Trump administration to confront the Kremlin, even as some of the President’s associates remain embroiled in an investigation over whether they colluded with Moscow to tip the 2016 U.S. election.

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Nikki Haley, Washington’s ambassador to the UN, said on CBS’s Face the Nation Sunday that she will roll out penalties for companies “that were dealing with equipment” connected to the Syrian chemical-weapons program. Ms. Haley, along with her British and French counterparts, will also press the UN to investigate the program and kick-start negotiations aimed at ending Syria’s bloody civil war.

Donald Trump, meanwhile, doubled down on his “Mission Accomplished!” tweet on the air strikes – even as it remains unclear that the bombing campaign will actually stop Mr. al-Assad from using gas again.

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The President took heat for using the phrase, which is heavily associated with a speech on May 1, 2003, by then-president George W. Bush, who declared victory in the U.S. invasion of Iraq while standing on an aircraft carrier with a banner emblazoned with the slogan behind him. Iraq, however, promptly descended into civil war and, 15 years later, U.S. troops are still there.

Mr. Trump hit back Sunday.

“The Syrian raid was so perfectly carried out, with such precision, that the only way the Fake News Media could demean was by my use of the term ‘Mission Accomplished,’” he tweeted. “I knew they would seize on this but felt it is such a great Military term, it should be brought back. Use often!”

In a Pentagon briefing Saturday, officials said the more than 100 missiles fired by the United States and its allies appeared to have got through to their targets and destroyed them. But they could not say exactly how much of Syria’s chemical-weapons stockpile had been eliminated.

U.S. President Donald Trump on Sunday defended his use of the phrase "mission accomplished" over the U.S.-led missile strikes on Syrian targets after it was seized on by the media. Reuters

Rex Brynen, a Middle East expert at McGill University, said the highly targeted strikes likely will not have much effect on the Syrian regime’s ability to use gas again. For one, he said, the Syrian army probably has chemical weapons in other locations. For another, it is not entirely clear that Mr. al-Assad even makes decisions on the use of such weapons personally: In the chaotic country, it is probable that regional commanders have their own supplies of gas and deploy them at will.

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Given this, Mr. Trump’s triumphalism risks backfiring catastrophically.

“It’s almost an invitation to the Syrians to do this again,” Mr. Brynen said. “They know the media would hang it around his neck.”

In a tense confrontation at the Security Council Saturday, U.S. Ambassador Haley pushed back against accusations from Russian envoy Vassily Nebenzia that “foreign intelligence services” had somehow faked the Douma attack.

“The pictures of dead children were not fake news,” shot back Ms. Haley, who emphasized that the United States would hit Mr. al-Assad again in the event of another chemical attack: “I spoke to the President this morning and he said, ’If the Syrian regime uses this poisonous gas again, the United States is locked and loaded.”

The Pentagon also warned of Russian efforts to spread false stories about the Douma attack or U.S. air strikes online, saying there had been a “2,000-per-cent increase in Russian trolls.” It was unclear exactly how the U.S. measured this. Moscow has been accused of employing “troll farms” – companies where people set up fake Twitter, Facebook and Instagram accounts to push out pro-Russia propaganda.

The trolls are central to special counsel Robert Mueller’s investigation into Russian efforts to tip the 2016 U.S. presidential election in Mr. Trump’s favour. His probe has zeroed in on several Trump campaign officials with ties to Moscow – and Mr. Trump has been criticized for expressing admiration for Russian President Vladimir Putin – but Mr. Trump denied any collusion between his circle and the Kremlin.

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Moscow on Sunday said Mr. Putin had spoken with his counterpart in Iran, Hassan Rouhani, who is also a Syrian ally. “Vladimir Putin, in particular, stressed that if such actions committed in violation of the UN Charter continue, then it will inevitably lead to chaos in international relations,” a Kremlin statement said.

British Prime Minister Theresa May, meanwhile, will address the House of Commons Monday to defend the attack and fend off critics who have argued she should have sought parliamentary approval for military action. Ms. May will argue that the U.K. and its allies are confident in their assessment that Syrian forces carried out the chemical attack and that failing to take action would have caused further suffering.

UN inspectors “have investigated previous attacks and on four occasions decided that the regime was indeed responsible,” Ms. May will tell MPs on Monday, according to officials in her office. “We are confident in our own assessment that the Syrian regime was highly likely responsible for this attack and that its persistent pattern of behaviour meant that it was highly likely to continue using chemical weapons. Furthermore, there were clearly attempts to block any proper investigation, as we saw with the Russian veto at the UN earlier in the week.”

Ms. May will also list the world leaders who have offered support, including Canadian Prime Minister Justin Trudeau.

French President Emmanuel Macron said Sunday he convinced Mr. Trump to stay in Syria and to limit military strikes to chemical facilities.

“Ten days ago, Donald Trump said that the United States wanted to disengage and we convinced him it was important to stay for the long term,” Mr. Macron said in a television interview. “We also convinced him to limit the strikes to the chemical capacities while there was a media uproar by way of tweets.”

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The French leader also argued that there was “complete international legitimacy” for the bombing because three of the five permanent members of the UN Security Council agreed.

Former U.S. secretary of state Madeleine Albright said Mr. Trump’s efforts to stand up to Mr. Putin are encouraging. But she worried that there isn’t a larger plan for what the White House is hoping to achieve in Syria.

“I am pleased now that Trump, who seemed to be entranced by the way that Putin behaved … has now recognized the fact that he is using his influence to support the criminal acts of Bashar al-Assad,” she said in an interview before the air strikes. “But we can’t just do one-off things without thinking through what the political settlement might look like, what are the various steps that have to be taken.”

Even if the bombing campaign does not achieve its mission of destroying Mr. al-Assad’s chemical weapons, Mr. Brynen said, it serves a larger purpose: Letting the world know that the use of chemical weapons will not go unchallenged, and telling other military leaders that they cannot follow the example of Damascus without repercussions.

“Normalizing the use of chemical weapons in insurgencies is not a good thing. This doesn’t really have to do with Syria specifically,” he said. “This has to do with spelling out to anyone who would use gas that there’s a cost.”