(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 war: Everything you need to know about how we got here
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.
February 2011: 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."
March 2011: Syrians protest in cities across the country, demanding the government enact reforms and release political protesters. Government forces fire on the demonstrations, killing dozens.
July 2011: As the government crackdown intensifies, a group of Syrian military officers defect and form the Free Syrian Army. In a video posted on YouTube, they say thousands of soldiers have left their posts instead of firing on protesters, and they promise to wage guerrilla war against regime forces. Three months later, a handful of political opposition groups establish the Syrian National Council, aimed at toppling President Bashar al-Assad.
August-October 2011: Top U.S. and European leaders call for Assad to step down, saying Syria's future "must be determined by its own people." But in October, Russia and China veto a U.N. Security Council resolution supported by the United States that would call for an immediate halt to violence and immediate sanctions.
December 2011 to February 2012: Suicide bombers kill 44 people in Damascus in blasts that bear the "blueprints of al Qaeda," the government says. In February the leader of al Qaeda, Ayman al-Zawahiri, praises Syrians for waging "jihad." The message comes as thousands of rebels join more extreme groups now operating in the country, including Jabhat al-Nusra, a group with close links to al Qaeda's affiliate in Iraq.
April 2012: Former United Nations chief Kofi Annan brokers a ceasefire plan calling for a halt to violence and the implementation of a political process to end the crisis, but the plan never gets off the ground.
June to July 2012: A top U.N. human rights official accuses Syria of engaging in crimes against humanity. A month later, the Zaatari refugee camp opens in Jordan to house the thousands of refugees streaming across the border from Syria.
August 2012: President Obama says President Assad will be crossing a "red line" if he uses chemical weapons against his own people in Syria. Syria has one of the largest and most advanced chemical warfare programs in the Arab world, according to experts.
February 2013: 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.
Spring 2013: The Islamic State in Iraq and Syria (ISIS) announces its arrival in northern Syria by seizing the city of Raqqa. The group began in 2004 as al Qaeda in Iraq before rebranding as ISIS two years later. The aim of ISIS -- led by Abu Bakr al-Baghdadi -- is to impose its strict Islamist ideology on Syrians in the areas it controls.
May 2013: The EU lifts its arms embargo on Syria, clearing the way for European nations to join Saudi Arabia, Qatar and other countries that have provided weapons or training to rebel groups in Syria. Russia, meanwhile, is shipping arms to the Syrian regime, and fighters from Hezbollah -- the Iranian-backed Lebanese militant group -- pour into the country to prop up the regime in Syria, a key ally.
August 2013: Hundreds of people are killed in a suspected chemical weapons attack in rebel-held areas on the outskirts of Damascus. Obama asks Congress to authorize military action against Syria.
September 2013: U.S. airstrikes seem inevitable. Asked what, if anything, the Syrian regime can do to stop an attack, U.S. Secretary of State John Kerry says Assad "could turn over every single bit of his chemical weapons to the international community in the next week." Russia immediately proposes putting Syria's chemical weapons under international control, a plan that staves off U.S. military action in the country for the time being.
February 2014: A second round of peace talks between the Syrian government, the opposition and an array of world powers ends without a solution. At least 140,000 Syrians are now dead, opposition groups say, and hundreds of thousands have been displaced.
June 2014: ISIS announces the establishment of a caliphate (Islamic state) stretching from western Syria to eastern Iraq. The group rebrands itself Islamic State and says Abu Bakr al-Baghdadi is the leader of this new state. The declaration comes after ISIS seizes Mosul, Iraq's second-biggest city.
August 2014: ISIS releases a video showing the beheading of American journalist James Foley in Raqqa, Syria. Foley is the first of several Western journalists and aid workers to be murdered by the group over the next few months, including Steven Sotloff, David Haines, Alan Henning and Peter Abdul-Rahman Kassig.
September 2014: American jets begin bombing ISIS targets in Syria, including the group's stronghold in Raqqa. The foreign partners participating in the strikes are Arab countries, including Saudi Arabia, Jordan, Bahrain and the United Arab Emirates.
January 2015: Kurdish fighters retake Kobani, a key Syrian town on the Turkish border, after nearly four months of fighting with ISIS.
May-August 2015: ISIS seizes Palmyra and blows up many of the ancient desert city's millennia-old temples and shrines. The group executes dozens of people in the city's ancient amphitheater, and beheads an antiquities expert who refused to reveal the location of other archaeological treasures.
September 30, 2015: After weeks of bulking up its military presence in Syria, Russia launches airstrikes on rebel targets in the country for the first time. Russia's intervention will prove decisive in swinging momentum in the conflict the Syrian regime's way.
November 24, 2015: Turkey shoots down a Russian Su-24 warplane over Syria, saying it had violated Turkish airspace. The incident triggers a war of words and escalating tensions between the two powers, who back opposite sides in the Syrian conflict.
December, 2015: The number of migrants who have entered Europe by sea and land in 2015 passes 1 million, the International Organization for Migration says, with about half of them Syrians fleeing the conflict. The surge in migrants poses a crisis for European nations, with governments split on how to respond.
February 4, 2016: More than $10 billion is pledged for Syria by international donors at a conference in London. But the goodwill is overshadowed a brutal Syrian government offensive, backed by Russian air power, on the northern city of Aleppo, which Turkey says has sent a fresh wave of refugees fleeing toward its border.
February 11, 2016: 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.