Armed forces from the UK, the US and France have fired missiles to destroy what they say are chemical weapons factories in Syria.
The missions took place overnight and hit places near to the Syrian capital, Damascus, as well as two locations near the city of Homs, according to the US.
It's the biggest military attack against Syria's President Bashar al-Assad's government by western powers in Syria's civil war.
Syria has been fighting a civil war for over seven years. Millions of ordinary people have been affected.
The strikes attacks come after claims of a chemical attack in an area called Douma which shocked a lot of people.
In conflicts, there are rules about what's acceptable - and chemical weapons are never supposed to be used.
Many think that the chemical attack in Douma was carried out by Syria's President Assad - but both he and the Russian government, who support him, say they didn't do it.
President Trump said: "The purpose of our actions tonight is to establish a strong deterrent against the production, spread, and use of chemical weapons".
UK Prime Minister Theresa May said it had been "right and legal" to take action, that the Syrian regime had demonstrated a "persistent pattern of behaviour" when it came to the use of chemical weapons, and that "must be stopped".
She added that using the armed forces was the most serious thing a Prime Minister could do, but she feels that the action taken by the UK, the USA and France sends "a clear message that the international community will not stand by and tolerate the use of chemical weapons."
However, many others don't agree that military strikes are the best approach and feel that the Prime Minister should have asked Members of Parliament to agree to action before it was taken.
The leader of the Labour Party, Jeremy Corbyn, said "Britain should be playing a leadership role to bring about a ceasefire in the conflict. Theresa May should have sought parliamentary approval (and) should do whatever possible to push Russia and the United States to agree to an independent UN-led investigation of last weekend's horrific chemical weapons attack so that those responsible can be held to account."
The strikes are over for now, but there was a clear warning from President Trump that if the Syrian government uses chemical weapons again then more missiles could be fired.
Russia have said that they would take any attacks very seriously and the world is waiting to see what will happen next.