Syria's war: How did we get here? | CNN

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(CNN) —

It began with the arrests of a handful of children in 2011. Since then, it’s exploded into the biggest humanitarian crisis since the Second World War.

Syria’s civil war has left more than 300,000 people dead and forced 11.2 million – nearly half the population – out of their homes. How did it come to this? Explore the events that have shaped the crisis since 2011.

It all starts with graffiti

Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded.

Photos: In photos: Syria's civil war

PHOTO: UNRWA/Reuters/Landov

Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded.

Photos: Explosions rock Ankara, Turkey

An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations.

Photos: In photos: Syria's civil war

PHOTO: Anwar Amro/AFP/Getty Images

An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations.

Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens.

Photos: In photos: Syria's civil war

PHOTO: Hussein Malla/AP

Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens.

Syrian children walk over bricks stored for road repairs during a spontaneous protest June 15, 2011, at a refugee camp near the Syrian border in Yayladagi, Turkey.

Photos: In photos: Syria's civil war

PHOTO: Vadim Ghirda/AP

Syrian children walk over bricks stored for road repairs during a spontaneous protest June 15, 2011, at a refugee camp near the Syrian border in Yayladagi, Turkey.

Jamal al-Wadi speaks in Istanbul on September 15, 2011, after an alignment of Syrian opposition leaders announced the creation of a Syrian National Council -- their bid to present a united front against Bashar al-Assad's regime and establish a democratic system.

Photos: In photos: Syria's civil war

PHOTO: Burhan Ozbilici/AP

Jamal al-Wadi speaks in Istanbul on September 15, 2011, after an alignment of Syrian opposition leaders announced the creation of a Syrian National Council -- their bid to present a united front against Bashar al-Assad's regime and establish a democratic system.

Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011.

Photos: In photos: Syria's civil war

PHOTO: Abdelhak Senna/AFP/Getty Images

Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011.

Supporters of al-Assad celebrate during a referendum vote in Damascus on February 26, 2012. Opposition activists reported at least 55 deaths across the country as Syrians headed to the polls. Analysts and protesters widely described the constitutional referendum as a farce. "Essentially, what (al-Assad's) done here is put a piece of paper that he controls to a vote that he controls so that he can try and maintain control," a US State Department spokeswoman said.

Photos: In photos: Syria's civil war

PHOTO: Khaled Al-Hariri/Reuters/LANDOV

Supporters of al-Assad celebrate during a referendum vote in Damascus on February 26, 2012. Opposition activists reported at least 55 deaths across the country as Syrians headed to the polls. Analysts and protesters widely described the constitutional referendum as a farce. "Essentially, what (al-Assad's) done here is put a piece of paper that he controls to a vote that he controls so that he can try and maintain control," a US State Department spokeswoman said.

Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012.

Photos: In photos: Syria's civil war

PHOTO: Bulent Kilic/AFP/Getty Images

Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012.

Rebel fighters with the Free Syrian Army capture a police officer in Aleppo, Syria, who they believed to be pro-regime militiaman on July 31, 2012. Dozens of officers were reportedly killed as rebels seized police stations in the city.

Photos: In photos: Syria's civil war

PHOTO: Emin Ozmen/AFP/Getty Images

Rebel fighters with the Free Syrian Army capture a police officer in Aleppo, Syria, who they believed to be pro-regime militiaman on July 31, 2012. Dozens of officers were reportedly killed as rebels seized police stations in the city.

A Free Syrian Army fighter runs for cover as a Syrian Army tank shell hits a building across the street during clashes in the Salaheddine neighborhood of central Aleppo on August 17, 2012.

Photos: In photos: Syria's civil war

PHOTO: Goran Tomasevic/Reuters/Landov

A Free Syrian Army fighter runs for cover as a Syrian Army tank shell hits a building across the street during clashes in the Salaheddine neighborhood of central Aleppo on August 17, 2012.

Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012.

Photos: In photos: Syria's civil war

PHOTO: Edouard Elias/Getty Images

Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012.

A Syrian man carrying grocery bags dodges sniper fire in Aleppo as he runs through an alley near a checkpoint manned by the Free Syrian Army on September 14, 2012.

Photos: In photos: Syria's civil war

PHOTO: Marco Longari/AFP/Getty Images

A Syrian man carrying grocery bags dodges sniper fire in Aleppo as he runs through an alley near a checkpoint manned by the Free Syrian Army on September 14, 2012.

Free Syrian Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012.

Photos: In photos: Syria's civil war

PHOTO: Marco Longari/AFP/Getty Images

Free Syrian Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012.

Smoke rises over the streets after a mortar bomb from Syria landed in the Turkish border village of Akcakale on October 3, 2012. Five people were killed. In response, Turkey fired on Syrian targets and its parliament authorized a resolution giving the government permission to deploy soldiers to foreign countries.

Photos: In photos: Syria's civil war

PHOTO: Rauf Maltas/Anadolu Agency/Reuters/LANDOV

Smoke rises over the streets after a mortar bomb from Syria landed in the Turkish border village of Akcakale on October 3, 2012. Five people were killed. In response, Turkey fired on Syrian targets and its parliament authorized a resolution giving the government permission to deploy soldiers to foreign countries.

A Syrian rebel walks inside a burnt section of the Umayyad Mosque in Aleppo hours before the Syrian army retook control of the complex on October 14, 2012.

Photos: In photos: Syria's civil war

PHOTO: Tauseef Mustafa/AFP/Getty Images

A Syrian rebel walks inside a burnt section of the Umayyad Mosque in Aleppo hours before the Syrian army retook control of the complex on October 14, 2012.

An Israeli tank crew sits on the Golan Heights overlooking the Syrian village of Breqa on November 6, 2012. Israel fired warning shots toward Syria after a mortar shell hit an Israeli military post. It was the first time Israel fired on Syria across the Golan Heights since the 1973 Yom Kippur War.

Photos: In photos: Syria's civil war

PHOTO: Menahem Kahana/AFP/Getty Images

An Israeli tank crew sits on the Golan Heights overlooking the Syrian village of Breqa on November 6, 2012. Israel fired warning shots toward Syria after a mortar shell hit an Israeli military post. It was the first time Israel fired on Syria across the Golan Heights since the 1973 Yom Kippur War.

Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012.

Photos: In photos: Syria's civil war

PHOTO: Javier Manzano/AFP/Getty Images

Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012.

The bodies of three children are laid out for identification by family members at a makeshift hospital in Aleppo on December 2, 2012. The children were allegedly killed in a mortar shell attack that landed close to a bakery in the city.

Photos: In photos: Syria's civil war

PHOTO: Javier Manzano/AFP/Getty Images

The bodies of three children are laid out for identification by family members at a makeshift hospital in Aleppo on December 2, 2012. The children were allegedly killed in a mortar shell attack that landed close to a bakery in the city.