"Mission Accomplished!" he wrote of the US, UK and French strikes against targets in Syria over the suspected use of chemical weapons. "Could not have had a better result".
His choice of words recalled a similar claim associated with President George W Bush following the US-led invasion of Iraq.
The Pentagon said Syria fired 40 surface-to-air missiles, but most were launched after the damage was done.
Russian Federation and Iran called the use of force by the United States and its allies a "military crime" and "act of aggression".
Trump's U.N. ambassador, Nikki Haley, told the session that the president has made it clear that if Assad uses poison gas again, "the United States is locked and loaded". In the bigger picture other interests and objectives are at play, and the U.S.is using the plight of the Syrian people to legitimise military intervention, making way for a possible shift in the U.S.'s Syria policy if the situation demands it.
Trump did the same on Friday, with allies France and Britain, in a response meant to deter Syrian President Bashar al-Assad from using chemical weapons again, but which was unlikely to change his grip on power. Three sites were targeted: Barzeh research and development centre in Damascus; Him Shinshar chemical weapons storage site, west of Homs; and Him Shinshar chemical weapons bunker, also west of Homs.
The airstrikes, which hit several sites, were a direct response to Assad's alleged use of chemical weapons in Douma on April 7. In a US military action a year ago in response to a sarin gas attack, the Pentagon said missiles took out almost 20 per cent of the Syrian air force.
Trump has made clear he wants to withdraw the roughly 2,000 USA troops in Syria involved in the anti-Islamic State campaign, and his administration has suspended support for Syrian rebels, evidence of his desire to disengage from Syria. The NATO alliance gave its full backing; NATO Secretary-General Jens Stoltenberg said the attack was about ensuring that chemical weapons can not be used with impunity.
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German Chancellor Angela Merkel called the attack "necessary and appropriate". "Use often!" he said. The words, which likely left many people in the USA government, congress, intelligence services and military with their heads in their hands, is ambiguous in every aspect: what was the "mission", and what does it mean for it to be "accomplished"? In fact, Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said no additional attacks were planned.
A USA official said that while top aides such as Defense Secretary Jim Mattis had persuaded Trump to avoid the tougher action the president initially wanted, arguing that that would have risked escalation with Assad ally Russian Federation, the administration is no closer to crafting a comprehensive strategy on the war in Syria.
What is certain is that Assad has been punished; a message has been sent without drawing the US closer into the conflict, but may set an important precedent.
Has this newfound mission to put an end to Assad's use of chemical weapons been "accomplished" by the airstrikes? "To some it said, well, Bush thinks the war in Iraq is over, when I didn't think that".
Vice-President Mike Pence, in Peru for a meeting of regional leaders, said "there will be a price to pay" involving military force if Syrian chemical weapons are used again.
"The Geneva Process hasn't worked and it's time to find something new or change it", said one US official.
Russian Federation called an emergency meeting of the United Nation's Security Council to introduce a resolution condemning the "aggression" of the joint strike from U.S., British and French forces.