Donald Trump has announced that a coalition of America, Britain and France have begun airstrikes against the Syrian regime to punish its chemical attack that killed more than 70 people.
In the televised address, Mr Trump said the allies would continue their response until Bashar Al-Assad, the Syrian president, ceased its use of chemical weapons.
Read Mr Trump's statement in full:
"My fellow Americans, a short time ago I ordered the United States armed forces to launch precision strikes on targets associated with the chemical weapon capabilities of Syrian dictator of Bashar al-Assad.
"A combined operation with the armed forces of France and the United Kingdom is now underway. We thank them both. Tonight I want to speak with you about why we have taken this action.
"One year ago, Assad launched a savage chemical weapons attack against his own innocent people. The United States responded with 58 missile strikes that destroyed 20% of the Syrian airforce.
"Last Saturday the Assad regime again deployed chemical weapons to slaughter innocent civilians, this time in the town of Douma near the Syrian capital of Damascus. This massacre was a significant escalation in a pattern of chemical weapons use by that very terrible regime. The evil and despicable attack left mothers and fathers, infants and children thrashing in pain and gasping for air. These are not the actions of a man, they are crimes of a monster instead.
"Following the horrors of World War One a century ago, civilised weapons joined together to ban chemical warfare. Chemical weapons are uniquely dangerous not only because they inflict gruesome suffering but because even small amounts can unleash widespread devastation.
"The purpose of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons. Establishing this deterrent is a vital national security interest of the United States. The combined American, British and French response to these atrocities will integrate all instruments of our national power: military, economic and diplomatic.
"We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents. I also have a message tonight for the two governments most responsible for supporting, equipping and financing the regime. To Iran and to Russia I ask - what kind of regime wants to be associated with the mass murder of innocent men, women and children?
"The nations of the world can be judged by the friends they keep. No state can succeed in the long run by promoting rogue states, brutal tyrants and murderous dictators. In 2013 President Putin and his government promised the world that they would guarantee the elimination of Syria's chemical weapons. Assad's recent attack and today's response are the direct result of Russia's failure to keep that promise. Russia must decide if it will continue down this dark path or if it will join with civilised nations as a force for stability and peace.Hopefully some day we'll get along with Russia and maybe even with Iran. But maybe not.
"I will say this, the United States has a lot to offer. With the greatest and most powerful economy in the history of the world, in Syria the United States, with but a small force being used to eliminate Isis, is doing what is necessary to protect the American people. Over the last year nearly 100 per cent of the territory once controlled by the so-called Isis caliphate in Syria and Iraq has been liberated and eliminated.
"The United States has also rebuilt our friendships across the Middle East, we have asked our partners to take greater responsibility for securing their home region including contributing large amounts of money for the resources, equipment and all of the anti-Isis effort. Increased engagement from our friends including Saudi Arabia, the United Arab Emirates, Qatar, Egypt and others can ensure that Iran cannot profit from the eradication of Isis.
"America does not seek an indefinite presence in Syria, under no circumstances. As other nations step up their contributions we look forward to the day when we can bring our warriors home, and great warriors they are. Looking around our troubled world, Americans have no illusions. We cannot purge the world of evil or act everywhere there is tyranny. No amount of American blood or treasure can produce lasting peace and security in the Middle East. It's a troubled place. We can try to make it better but it is a troubled place. The United States will be a partner and a friend, but the fate of the region lies in the hands of its own people.
"In the last century we looked into the darkest places of the human soul. We saw the anguish that can be unleashed and the evil that can take hold. By the end of World War One more than one million people had been killed or injured by chemical weapons. We never want to see that ghastly spectre return.
"So today the nations of Britain, France and the United States of America have marshalled their righteous power against barbarism and brutality. Tonight I ask all Americans to say a prayer for our noble warriors and our allies as they carry out their missions, We pray that God will bring comfort to those suffering in Syria, we pray that God will guide the whole region toward a future of dignity and of peace and we pray that God continue to watch over and will bless the United States of America. Thank you and good night."
Read Prime Minister Theresa May's statement:
"This evening I have authorised British armed forces to conduct co-ordinated and targeted strikes to degrade the Syrian Regime’s chemical weapons capability and deter their use. We are acting together with our American and French allies. In Douma, last Saturday a chemical weapons attack killed up to 75 people, including young children, in circumstances of pure horror.
"The fact of this attack should surprise no-one. The Syrian Regime has a history of using chemical weapons against its own people in the most cruel and abhorrent way. And a significant body of information including intelligence indicates the Syrian Regime is responsible for this latest attack. This persistent pattern of behaviour must be stopped – not just to protect innocent people in Syria from the horrific deaths and casualties caused by chemical weapons but also because we cannot allow the erosion of the international norm that prevents the use of these weapons.
"We have sought to use every possible diplomatic channel to achieve this. But our efforts have been 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.
"This is not about intervening in a civil war. It is not about regime change. It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties. And while this action is specifically about deterring the Syrian Regime, it will also send a clear signal to anyone else who believes they can use chemical weapons with impunity.
"At this time, my thoughts are with our brave British servicemen and women – and our French and American partners – who are carrying out their duty with the greatest professionalism. The speed with which we are acting is essential in co-operating with our partners to alleviate further humanitarian suffering and to maintain the vital security of our operations.
"This is the first time as Prime Minister that I have had to take the decision to commit our armed forces in combat – and it is not a decision I have taken lightly. I have done so because I judge this action to be in Britain’s national interest. We cannot allow the use of chemical weapons to become normalised – within Syria, on the streets of the UK, or anywhere else in our world. We would have preferred an alternative path. But on this occasion there is none.
"History teaches us that the international community must defend the global rules and standards that keep us all safe. That is what our country has always done. And what we will continue to do."