Before the strike, Trump was against military action aimed at Assad's forces. What led to the US President's change of heart? Here are some key details to get you up to speed:
Warplanes bombed the center of rebel-held Khan Sheikhoun that day, witnesses said. People around the initial blast area appeared to be exposed to neurotoxic agents "such as sarin gas or similar compounds," international aid group Doctors Without Borders said.
About five hours after the initial attack, two other locations -- a makeshift clinic and Al-Rahma hospital, where the injured were being triaged -- were hit in fresh strikes, Idlib health officials said, though there were no descriptions of gas then.
At least 86 people, including 26 children, were killed in Tuesday's attacks, the Union of Medical Care and Relief Organizations said.
The Syrian government denied using chemical weapons, but several countries, including the United States, said they believed the regime was responsible for the attack.
The United States is trying to determine whether others were complicit
. For example, the Pentagon is looking at whether a Russian warplane dropped the bomb that hit the hospital, and whether the purpose of the strike was to destroy evidence of the chemical attack, a senior US defense official said Friday.
Images of lifeless children from the Khan Sheikhoun attack
became yet another symbol of the international community's failure to help end the bloody civil war.
In one particular photo, a sobbing father clung to bodies of his 9-month-old twins. Others showed children gasping for breath, along with shell-shocked relatives also affected by the gas.
As photos from the attack's aftermath circulated on social media, so did the outrage.
Trump was not immune to the horror either and admitted the images changed his thinking on Syria.
"When you kill innocent children -- innocent babies -- babies -- little babies with a chemical gas that is so lethal, people were shocked to hear what gas it was, that crosses many, many lines," the US leader said.
Until the strike, Trump gave no indication of what he was planning to do. Then everything changed early Friday in Syria (Thursday night in the United States).
"Tonight, I ordered a targeted military strike on the air base in Syria from where the chemical attack was launched," he said from his Mar-a-Lago estate in Florida. "It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons."
The US missiles struck the Syrian government's Shayrat air base, where the warplanes believed to have carried out Tuesday's attack were based, US officials said.
Details about how many people died in the US strike were sketchy. Syrian state-run media reported that the strike killed nine people, including four children. Five people were killed in one village near the air base, and four others people died in an another village, the state-run Syrian Arab News Agency said.
Earlier, Syria's military said six people were killed. It wasn't immediately clear whether the military's toll and SANA's numbers overlapped.
The United States started launching strikes in Syria in September 2014 under President Barack Obama, but it says it had targeted only the ISIS terrorist group and not government forces.
In September, US-led coalition strikes
killed more than 60 Syrian soldiers near Deir Ezzor Airport, the Russian military said. The US said the strikes were intended to target ISIS and expressed regret over the deaths.
The United States has "a very high level of confidence" that Syrian government aircraft carried out attacks involving sarin nerve gas not only on Tuesday in Khan Sheikhoun but also on March 25 and 30 in Hama province, US Secretary of State Rex Tillerson said
The US military also shared an image of a radar track of a Syrian airplane from the Shayrat airfield flying to the purported chemical strike area Tuesday.
Syria has continuously blamed such attacks on terrorists.
WHAT WAS HIT?
An initial assessment found 58 of the 59 missiles "severely degraded or destroyed" their intended target, a US defense official said on condition of anonymity.
The strike occurred during minimal activity at the base and targeted various items, including petroleum and logistical storage, ammunition supply bunkers and air defense systems, the Pentagon said.
Syria described the US strike as an "erroneous American strategy."
"It makes the United States of America a partner of ISIS, Nusra Front and other terrorist organizations who .... have been attacking Syrian army positions and Syrian military bases," an armed forces' statement said.
The Syrian Foreign Ministry described the attack as "a pretext promoted by terrorist organizations and its operatives in Washington, Tel Aviv, Riyadh, Doha, Ankara, London and Paris and their media outlets."
Assad called it an "unjust and unabashed assault."
Syria's powerful backer, Russia, did not mince words.
The Kremlin said Russian President Vladimir Putin regards the US strike as "an aggression against a sovereign state in violation of the norms of international law."
"Vladimir Putin believes that complete disregard for factual information about the use by terrorists of chemical weapons drastically aggravates the situation," its statement said.
It said the strike "dealt a serious blow to Russian-US relations, which are already in a poor state."
Russia said Friday it was suspending a communications channel aimed at minimizing risks of in-flight incidents between the US-led coalition and Russian aircraft.
The Russian Defense Ministry also said that it plans to bolster and increase the effectiveness of the Syrian military's air defense system.
Before the strike, Russian officials warned the United States against making any "snap judgments" on Tuesday's chemical weapons attack.
A number of countries, including Syria's neighbor, Turkey, had a different reaction
"Turkey will fully support the steps that will be taken to ensure accountability and to prevent impunity in case of such crimes," it said.
"As a country hosting 3 million Syrians, Turkey will continue its cooperation with the international community to prevent the regime's continued practices of terror and mass punishment."
Saudi Arabia's King Salman was briefed Friday by Trump about the missile strikes. "On his part, the Custodian of the Two Holy Mosques congratulated the US President on this courageous decision which serves the regional and world interest," the Saudi Press Agency said.
NATO Secretary General Jens Stoltenberg said the Syrian regime "bears the full responsibility" for the US strike.
"Any use of chemical weapons is unacceptable, cannot go unanswered, and those responsible must be held accountable," Stoltenberg said.
China, at a UN Security Council meeting Friday, called for nations to use diplomatic channels rather than military action to solve the crisis in Syria.
"Military means will not work," said Liu Jieyi, China's envoy to the United Nations. "It will only worsen the suffering of the Syrian people."
One of Assad's enemies in the civil war supported the US action.
Col. Fateh Hassoun of the Free Syrian Army said in a statement: "American and its allies have stood up for the values of freedom and humanity today in Syria. The war criminal Bashar al-Assad understands only the language of force. Today's airstrikes on regime military assets are a welcome first step, but must now be followed by further action to deter and prevent all attacks against civilians, and pave the way for a genuine political solution including the departure of Assad."