Key moments in the Syrian conflict that has killed over 400,000

The conflict in Syria has been raging since 2011. It has not only cost the lives of over 400,000 Syrians and created the biggest refugee crisis of our time but has involved a large cast of domestic and international actors.

Here are some of the key moments in the conflict.

Spring 2011: Civilian protests topple the regimes of Tunisian President Zine El Abidine Ben Ali and Egyptian President Hosni Mubarak.

The Arab Spring sparks demonstrations in Syria, where President Bashar al-Assad responds with punitive action, killing and imprisoning dissenting civilians.

The crackdown spurs the formation of the Free Syrian Army, which aims to overthrow Assad's government.

Aug. 20, 2012: Then-U.S. president Barack Obama draws a "red line" at the use of chemical weapons, telling reporters there will be "enormous consequences" for the Syrian government following deployment of weapons such as sarin and chlorine gas.

But Obama later said he would seek approval from a skeptical Congress for a military strike in Syria. That approval never happened.

March 19, 2013: A government-controlled area of Aleppo is hit by a rocket containing sarin.

The government blames opposition forces for the attack, which kills at least 26 people.

A UN investigation cannot confirm the perpetrator, but says in a report that those responsible for the attack "likely had access to the chemical weapons stockpile of the Syrian military."

Aug. 21, 2013: Opposition-held areas in Damascus are attacked by ground missiles containing sarin.

Death estimates vary. U.K. intelligence says at least 350 people died, while US intelligence places the toll at about 1,400.

Western states believe Assad is behind the attack, but that remains unconfirmed by the UN.

Sept. 14, 2013: The U.S. and Russia agree to a deal requiring Syria to destroy its arsenal of deadly chemical weapons.

An international watchdog says Syria ships out the last of its declared weapons in June 2014.

Chlorine gas supplies were not included in the agreement. The UN later confirms the use of chlorine gas on civilians by the Syrian military.

Sept. 22, 2014: A US-led coalition that includes Saudi Arabia and later Britain, France and Germany begins airstrikes on ISIS targets across Syria.

Sept. 2, 2015: Alan Kurdi, a three-year-old Syrian refugee, drowns during an attempted crossing to Europe from Turkey.

This photo makes international headlines, sparking conversation about Syrian refugees and the often perilous journeys they face trying to escape conflict.

Sept. 30, 2015: Russia joins the bombing campaign, targeting ISIS and western-backed opposition groups.

April 6, 2017: U.S. President Donald Trump orders the first direct attack by the U.S. military against the Assad government in response to Tuesday's chemical attacks that killed over 70 civilians

Warships stationed in the Mediterranean launch 59 missiles, which strike the Shayrat air base at 3:40 a.m. local time.

The attack kills nine people, according to the Syrian government.

To date: Over 12 million Syrians have been displaced, five million of them to other countries.