(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.









it all starts with graffiti









a sandstorm blows over damaged buildings in the rebel-held area of douma, east of damascus, on september 7, 2015.









a sandstorm blows over damaged buildings in the rebel-held area of douma, east of damascus, on september 7, 2015.









a man's body lies in the back of van as people search for the injured after airstrikes allegedly by the syrian government on a market in a rebel-held eastern ghouta town on august 31, 2015.









a man's body lies in the back of van as people search for the injured after airstrikes allegedly by the syrian government on a market in a rebel-held eastern ghouta town on august 31, 2015.









a refugee carries mattresses as he re-enters syria from turkey on june 22, 2015, after kurdish people's protection units regained control of the area around tal abyad, syria, from isis.









a refugee carries mattresses as he re-enters syria from turkey on june 22, 2015, after kurdish people's protection units regained control of the area around tal abyad, syria, from isis.









a syrian child fleeing the war gets lifted over fences to enter turkish territory illegally near a border crossing at akcakale, turkey, on june 14, 2015.









a syrian child fleeing the war gets lifted over fences to enter turkish territory illegally near a border crossing at akcakale, turkey, on june 14, 2015.









a syrian boy receives treatment at a local hospital following an alleged chlorine gas attack in the idlib suburb of jabal al-zawia on april 27, 2015.









a syrian boy receives treatment at a local hospital following an alleged chlorine gas attack in the idlib suburb of jabal al-zawia on april 27, 2015.









nusra front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held idlib countryside on march 22, 2015.









nusra front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held idlib countryside on march 22, 2015.









rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of hama on march 9, 2015.









rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of hama on march 9, 2015.









a man gives medical assistance as two wounded children wait nearby at a field hospital in douma on february 2, 2015.









a man gives medical assistance as two wounded children wait nearby at a field hospital in douma on february 2, 2015.









a long-exposure photograph shows a rocket being launched in aleppo on october 5, 2014.









a long-exposure photograph shows a rocket being launched in aleppo on october 5, 2014.









medics tend to a man's injuries at a field hospital in douma after airstrikes on september 20, 2014.









medics tend to a man's injuries at a field hospital in douma after airstrikes on september 20, 2014.









volunteers remove a dead body from under debris after shelling in aleppo on august 29, 2014. according to the syrian civil defense, barrel bombs are now the greatest killer of civilians in many parts of syria. the white helmets are a humanitarian organization that tries to save lives and offer relief.









photographs of victims of the assad regime are displayed as a syrian army defector known as "caesar," center, appears in disguise to speak before the house foreign affairs committee in washington. the july 31, 2014, briefing was called "assad's killing machine exposed: implications for u.s. policy." caesar, apparently a witness to the regime's brutality, has smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. cnn cannot independently confirm the authenticity of the photos, documents and testimony referenced in the report.









rebel fighters execute two men on july 25, 2014, in binnish, syria. the men were reportedly charged by an islamic religious court with detonating several car bombs.









rebel fighters execute two men on july 25, 2014, in binnish, syria. the men were reportedly charged by an islamic religious court with detonating several car bombs.









a giant poster of al-assad is seen in damascus on may 31, 2014, ahead of the country's presidential elections. he received 88.7% of the vote in the country's first election after the civil war broke out.









a giant poster of al-assad is seen in damascus on may 31, 2014, ahead of the country's presidential elections. he received 88.7% of the vote in the country's first election after the civil war broke out.









a free syrian army fighter fires a rocket-propelled grenade during heavy clashes in aleppo on april 27, 2014.









a free syrian army fighter fires a rocket-propelled grenade during heavy clashes in aleppo on april 27, 2014.









a u.s. ship staff member wears personal protective equipment at a naval airbase in rota, spain, on april 10, 2014. a former container vessel was fitted out with at least $10 million of gear to let it take on about 560 metric tons of syria's most dangerous chemical agents and sail them out to sea, officials said.









a u.s. ship staff member wears personal protective equipment at a naval airbase in rota, spain, on april 10, 2014. a former container vessel was fitted out with at least $10 million of gear to let it take on about 560 metric tons of syria's most dangerous chemical agents and sail them out to sea, officials said.









a man holds a baby who was rescued from rubble after an airstrike in aleppo on february 14, 2014.









a man holds a baby who was rescued from rubble after an airstrike in aleppo on february 14, 2014.









residents wait to receive food aid distributed by the u.n. relief and works agency at the besieged al-yarmouk camp, south of damascus, on january 31, 2014.









residents wait to receive food aid distributed by the u.n. relief and works agency at the besieged al-yarmouk camp, south of damascus, on january 31, 2014.









an injured man is helped following an airstrike in aleppo's maadi neighborhood on december 17, 2013.









an injured man is helped following an airstrike in aleppo's maadi neighborhood on december 17, 2013.









syrian children wait as doctors perform medical checkups at a refugee center in sofia, bulgaria, on october 26, 2013.









syrian children wait as doctors perform medical checkups at a refugee center in sofia, bulgaria, on october 26, 2013.









residents run from a fire at a gasoline and oil shop in aleppo's bustan al-qasr neighborhood on october 20, 2013. witnesses said the fire was caused by a bullet from a pro-government sniper.









residents run from a fire at a gasoline and oil shop in aleppo's bustan al-qasr neighborhood on october 20, 2013. witnesses said the fire was caused by a bullet from a pro-government sniper.









the u.n. security council passes a resolution september 27, 2013, requiring syria to eliminate its arsenal of chemical weapons. al-assad said he would abide by the resolution.









the u.n. security council passes a resolution september 27, 2013, requiring syria to eliminate its arsenal of chemical weapons. al-assad said he would abide by the resolution.









a handout image released by the syrian opposition's shaam news network shows people inspecting bodies of children and adults who rebels claim were killed in a toxic gas attack by pro-government forces on august 21, 2013. a week later, u.s secretary of state john kerry said u.s. intelligence information found that 1,429 people were killed in the chemical weapons attack, including more than 400 children. al-assad's government claimed that jihadists fighting with the rebels carried out the chemical weapons attacks to turn global sentiments against it.









an aerial view shows the zaatari refugee camp near the jordanian city of mafraq on july 18, 2013.









an aerial view shows the zaatari refugee camp near the jordanian city of mafraq on july 18, 2013.









rebels launch a missile near the abu baker brigade in al-bab, syria, on january 16, 2013.









rebels launch a missile near the abu baker brigade in al-bab, syria, on january 16, 2013.









syrians look for survivors amid the rubble of a building targeted by a missile in the al-mashhad neighborhood of aleppo on january 7, 2013.









syrians look for survivors amid the rubble of a building targeted by a missile in the al-mashhad neighborhood of aleppo on january 7, 2013.









a rebel fighter prepares the wires of a car-mounted camera used to spy on syrian government forces while his comrade smokes a cigarette in aleppo's bab al-nasr district on january 7, 2013.









a rebel fighter prepares the wires of a car-mounted camera used to spy on syrian government forces while his comrade smokes a cigarette in aleppo's bab al-nasr district on january 7, 2013.









a father reacts after the deaths of two of his children in aleppo on january 3, 2013.









a father reacts after the deaths of two of his children in aleppo on january 3, 2013.









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.









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.









smoke rises in the hanano and bustan al-basha districts in aleppo as fighting continues through the night on december 1, 2012.









smoke rises in the hanano and bustan al-basha districts in aleppo as fighting continues through the night on december 1, 2012.









rebels celebrate next to the remains of a syrian government fighter jet that was shot down at daret ezza, on the border of the provinces of idlib and aleppo, on november 28, 2012.









rebels celebrate next to the remains of a syrian government fighter jet that was shot down at daret ezza, on the border of the provinces of idlib and aleppo, on november 28, 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.









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.









relatives of syrian detainees who were arrested for participating in anti-government protests wait in front of a police building in damascus on october 24, 2012. the syrian government said it released 290 prisoners.









relatives of syrian detainees who were arrested for participating in anti-government protests wait in front of a police building in damascus on october 24, 2012. the syrian government said it released 290 prisoners.









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.









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.









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.









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.









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.









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.









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.









family members mourn the deaths of their relatives in front of a field hospital in aleppo on august 21, 2012.









family members mourn the deaths of their relatives in front of a field hospital in aleppo on august 21, 2012.









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.









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.









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.









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.









people gather on may 26, 2012, at a mass burial for victims reportedly killed by syrian forces in syria's houla region. u.n. officials confirmed that more than 100 syrian civilians were killed , including nearly 50 children. syria's government denied its troops were behind the bloodbath.









an injured man gets treated in a damascus neighborhood on april 3, 2012.









an injured man gets treated in a damascus neighborhood on april 3, 2012.









syrian refugees walk across a field in syria before crossing into turkey on march 14, 2012.









syrian refugees walk across a field in syria before crossing into turkey on march 14, 2012.









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 u.s. state department spokeswoman said.









suicide bombs hit two security service bases in damascus on december 23, 2011, killing at least 44 people and wounding 166.









jamal al-wadi of daraa 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 al-assad's regime and establish a democratic system.









jamal al-wadi of daraa 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 al-assad's regime and establish a democratic system.









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.









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.









anti-government protesters demonstrate in daraa on march 23, 2011. in response to continuing protests, the syrian government announced several plans to appease citizens.









anti-government protesters demonstrate in daraa on march 23, 2011. in response to continuing protests, the syrian government announced several plans to appease citizens.









an injured man lying in the back of a vehicle is rushed to a hospital in daraa, south of damascus, 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.









an injured man lying in the back of a vehicle is rushed to a hospital in daraa, south of damascus, 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.









pro-government protesters hold pictures of syrian president bashar al-assad and his late father, hafez al-assad, during a rally in damascus, syria, on march 18, 2011. bashar al-assad has ruled syria since 2000, when his father passed away following 30 years in charge. an anti-regime uprising that started in march 2011 has spiraled into civil war. the united nations estimates more than 220,000 people have been killed.









pro-government protesters hold pictures of syrian president bashar al-assad and his late father, hafez al-assad, during a rally in damascus, syria, on march 18, 2011. bashar al-assad has ruled syria since 2000, when his father passed away following 30 years in charge. an anti-regime uprising that started in march 2011 has spiraled into civil war. the united nations estimates more than 220,000 people have been killed.









as arab spring demonstrations overthrow governments across the middle east, a group of children in daraa, southern syria, are arrested and allegedly tortured for scrawling graffiti on a school reading "the people want to topple the regime."









outrage over the arrests turns into protests and a brutal crackdown









protesters clash with syrian security forces on the streets of daraa in march 2011.









as the bloodshed gets worse, the opposition gets organized









the west calls for assad to quit, but russia and china have other ideas









meanwhile, the influence of islamist groups grows in rebel ranks









the u.n. brokers a ceasefire that falls apart almost immediately









as refugees flee, the u.n. accuses syria of crimes against humanity









barack obama announces his 'red line' in syria for the first (but not last) time









two years in, 60,000 are dead, and the u.s. says it will send aid to rebels









february 2013: the u.s. the u.s. promises to send food and medical supplies -- but not weapons -- to syrian rebels. it's the first such move since the conflict began two years before, in an effort to hem in the radical islamist groups vying for influence in syria. more than 60,000 people are now dead and nearly a million have fled the country.









european nations join the list of countries sending weapons into syria









syria crosses obama's 'red line,' and the u.s. prepares to attack









john kerry goes off script, and the russians pounce









140,000 are dead in syria as peace talks go nowhere fast









isis declares an independent state in iraq and syria









the u.s. launches airstrikes on isis in syria









kurdish forces, backed by u.s. strikes, take back kobani from isis









isis seizes palmyra and blows up its priceless ruins









donors pledge more than $10 billion to syria









february 11, 2016: diplomats from more than a dozen countries, including the united states and russia, diplomats from more than a dozen countries, including the united states and russia, agree in munich, germany, to a "cessation of hostilities" -- a temporary halt in fighting that commonly happens at the start of a peace process -- and to the delivery of aid. terrorist groups, including isis and al-nusra, are not included in the deal. the partial truce does not take hold a week later as originally hoped, but there are renewed efforts to implement the deal the following friday
syria's war: how did we get here?
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