Here’s what we know about the strikes.
Who and why?
The strikes were conducted by the US, the UK and France following a suspected chemical attack on the town of Douma, which left an estimated 70 people dead. Russia and Syria said the attack was fabricated.
In a televised address from the White House, US President Donald Trump said: "A short time ago, I ordered the United States Armed Forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al-Assad."
“A combined operation with the armed forces of France and the United Kingdom is now under way. We thank them both. This massacre was a significant escalation in a pattern of chemical weapons used by that very terrible regime.”
He said the mission was to “establish a strong deterrent against the production, spread, and use of chemical weapons,”as he criticised Iran and Russia for supporting Assad.
UK Prime Minister Theresa May said the strikes were not about "regime change," but "about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties."
She said it sent a message to anyone who used chemical weapons, and came after efforts to use diplomatic channels were “repeatedly thwarted.”
“Even this week the Russians vetoed a Resolution at the UN Security Council which would have established an independent investigation into the Douma attack. So there is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian Regime.”
In a speech on Saturday morning, she said it was "both right and legal" to take military action.
May said she will address the issue in parliament on Monday.
French President Emmanuel Macron said France had joined the US and UK because “we cannot tolerate the recurring use of chemical weapons, which is an immediate danger for the Syrian people and our collective security.”
"The red line set by France in May 2017 has been crossed," he said.
US Defence Secretary James Mattis and Marine General Joseph Dunford said three main chemical weapons facilities were targeted at 9 pm EST (1 am CET) by missiles from both the sea and aircraft.
The strikes targeted:
The Barzah chemical weapons research and development centre near Damascus
The Him Shinshar chemical weapons storage facility near Hom
The Him Shinshar chemical weapons bunker facility near Homs
In total, the US and its allies fired more than 100 missiles.
The Pentagon said on Saturday that the strikes had successfully hit every target and were aimed to deliver an unambiguous signal to the Syrian government and deter the future use of chemical weapons.
It said the strike was designed to degrade Syria's chemical weapons capability without killing civilians or foreign fighters, noting that it was not aware of any civilian casualties.
Weapons used in the strike included:
US Tomahawk cruise missiles, warships and submarines - Three US warships and one submarine using Tomahawk missiles participated in the strikes, Pentagon officials said. Guided-missile destroyer USS Monterey fired 30 Tomahawks, the USS Laboon launched seven and the USS Higgins fired 23. Attack submarine USS John Warner fired six missiles.
US B-1B bombers - Two B-1B bombers were used in the strike, the Pentagon said. The bombers fired 19 JASSM cruise missiles.
UK Tornado and Typhoon fighters - The UK contributed four Tornado fighter jets armed with Storm Shadow cruise missiles. Officials later said that Typhoon fighters had also been used. Pentagon officials said the British jets fired a total of eight Storm Shadow missiles.
French Rafale and Mirage jets - Video footage was shared by Macron's office showing Rafale fighter jets taking off for the mission. US officials said Mirage jets were also used, and that French aircraft fired nine missiles in total.
French frigates and cruise missiles - France fired three cruise missiles from one of its multi-mission frigates, according to the Pentagon.
The US ambassador to the UN said President Donald Trump told her that if the Syrian regime uses poisonous gas again "the US is locked and loaded" to strike again during a Security Council emergency meeting on Saturday.
Mattis said the joint strikes against Syria were a "one-time shot" unless Assad continues to use chemical weapons.
German Foreign Minister Heiko Maas said on Saturday Germany would join France in efforts to achieve a lasting ceasefire in Syria, adding that Germany would use its ties with Russia to ensure it adopted a "constructed" stance to the issue.
Germany, France, UK, and the US will meet in London on Sunday to discuss the next steps after the air strikes.
A senior official in the regional alliance that has supported Assad in the Syrian war told Reuters the attack on Syria will be seen as limited if it is now over and there is no second round.
Syrian and Russian response
Syrian state TV described the strikes as a “flagrant violation” of international law. It said air defences confronted the attack and had shot down 13 missiles in the Kiswah area south of Damascus.
Russia's ambassador to the UN said the "US and allies' attack was an act of aggression against a sovereign state," adding that they had "demonstrated a blatant disregard for international law."
Russia’s ambassador to the US warned that there would be consequences for the strike.
“The worst apprehensions have come true. Our warnings have been left unheard...We warned that such actions will not be left without consequences,” Ambassador Anatoly Antonov said in a statement.
“All responsibility for them rests with Washington, London and Paris. Insulting the President of Russia is unacceptable and inadmissible. The US - the possessor of the biggest arsenal of chemical weapons - has no moral right to blame other countries.”
Russia's Foreign Ministry described the strikes as an "outrageous violation" of international law.”
Deputy Foreign Minister Sergei Ryabkov said Moscow is in touch with the countries that took part in the strike, RIA news agency reported.
Russia resolution rejected
Members of the UN Security Council rejected a Russian resolution calling for condemnation of the military strikes.
The Russian measure would have condemned the “aggression” against Syria and demanded that the three allies refrain from further strikes.
Only Russia, China, and Bolivia voted in favour of the resolution at the end of an emergency meeting of the 15-member council called by Russia on Saturday.
Iran’s Foreign Ministry strongly condemned the airstrikes and warned of its regional consequences.
Supreme Leader Ayatollah Ali Khamenei said the strikes were a crime and would not achieve any gains.
"US, allies will not gain any achievements from crimes in Syria. Attacking Syria is a crime. US president, UK prime minister and the president of France are criminals," Khamenei said in a speech cited by Iranian TV.
Turkey welcomed the air strikes on the Syrian government as an “appropriate” response, a foreign ministry source told Reuters.
A spokesman for the ruling Justice and Development Party said Turkey was informed ahead of the action.
In a statement, Ankara said that Turkish President Tayyip Erdogan had talked to Russian President Vladimir Putin on the phone and that both had agreed to "work together to reduce tension and continue efforts for a political solution."
NATO said all 29 members in the alliance backed. NATO Secretary General Jens Stoltenberg said the strikes were about making sure that chemical weapons were not "used without impunity," adding that the action by the allies was "very successful" and it "significantly degraded the abilities of Syrian President Bashar Assad's forces to launch chemical attacks again."
"This will reduce the regime’s ability to further attack the people of Syria with chemical weapons,” said NATO chief Jens Stoltenberg in a statement.
Jeremy Corbyn, leader of the opposition Labour Party, described the airstrikes as “legally questionable".
"Bombs won’t save lives or bring about peace. This legally questionable action risks escalating further, as US defence secretary James Mattis has admitted, an already devastating conflict and therefore makes real accountability for war crimes and use of chemical weapons less, not more likely," he said.
European Council President Donald Tusk said the strikes made it clear "that the Syrian regime together with Russia and Iran cannot continue this human tragedy, at least not without cost."
"The EU will stand with our allies on the side of justice," he wrote on Twitter.
The European Commission issued a statement, condemning the use of chemical weapons in the "strongest terms".
"The international community has the responsibility to identify and hold accountable those responsible of any attack with chemical weapons," it said.
"This was not the first time that the Syrian regime has used chemical weapons against civilians but it must be the last."
Egypt's Foreign Ministry said it was deeply concerned "at the current military escalation in Syria". It also rejected the use of internationally banned weapons on Syrian territory and demanded a transparent international investigation.
"The Arab Republic of Egypt expresses its solidarity with the brotherly Syrian people in their aspirations to live in security and stability," the ministry said in a statement.
It called for a peaceful resolution to the Syrian crisis and efforts to ensure humanitarian aid reaches those affected by the conflict.
China’s Foreign Ministry said it believes a political settlement is the only way to resolve the Syrian issue, as it called for a full, fair and objective investigation into suspected chemical weapon attacks in Syria.
Ministry spokesperson Hua Chunying said China has consistently opposed the use of force in international relations and that any military action that bypassed the United Nations’ Security Council violated the principles and basic norms of international law.
A Foreign Ministry statement said Pakistan was following the situation in Syria with "grave concern" and calls on all sides to refrain from violating the UN Charter.
Prime Minister Shinzo Abe said Japan fully supports the strikes in Syria.
"The use of chemical weapons is inhumane and can absolutely not be allowed. We fully support the US, British and French stance of not permitting the use and spread of chemical weapons. We also understand that this measure was taken as one to prevent the further degradation of the situation. We will gather the NSC (National Security Council) tonight to analyse the situation and consider what we do next," he said.
Canadian President Justin Trudeau said the country supports the decision “to take action to degrade the Assad regime's ability to launch chemical weapons attacks against its own people.”
"We will continue to work with our international partners to further investigate the use of chemical weapons in Syria. Those responsible must be brought to justice," he said.
German Foreign Minister Heiko Maas said Germany would join France in efforts to push for a new international effort to achieve a lasting ceasefire in Syria.
Spain's Foreign Ministry tweeted they found the air strikes by the US and its allies "legitimate and proportionate."
Belgium's Prime Minister tweeted that Belgium showed "understanding" for the action by the US, France, and the UK.
The Danish Foreign Ministry tweeted that it "supported the American-led reaction from our allies to the chemical attack in Syria."
Israel commended the swift response of the US and its allies.
"Last year, President Trump made clear that the use of chemical weapons crosses a red line. Tonight, under American leadership, the United States, France and the United Kingdom enforced that line.
“Syria continues to engage in and provide a base for murderous actions, including those of Iran, that put its territory, its forces and its leadership at risk," an Israeli official told NBC News.
Australia said it fully supports the US-led strikes.
"The use of chemical weapons by anyone, anywhere, under any circumstances, is illegal and reprehensible. The Assad regime cannot be allowed to commit such crimes with impunity. That is why Australia fully supports the US-led strikes against chemical weapons facilities in Syria today," said Minister of Defence Marise Payne.
"The strike, by the United States, France, and the United Kingdom forces, targeted the Syrian regime's chemical weapons research, development, and production capability. Those locations were specifically chosen to minimise the risk to civilians."
Iraq's foreign ministry said the air strikes marked "a very dangerous development" that could give terrorism an opportunity to expand in the region.
“Such action could have dangerous consequences, threatening the security and stability of the region and giving terrorism another opportunity to expand after it was ousted from Iraq and forced into Syria to retreat to a large extent,” it said in a statement.
The ministry called on Arab leaders to discuss the situation at a summit due to be held in Saudi Arabia on Sunday.
Protests against Syria air strikes erupt outside White House
Several dozen drotesters gathered outside the White House in Washington D.C to condemn the Western air raids on Syria.
Protesters chanted "hands off Syria" and called for peace, not war.