Here is some background information about the ongoing civil war in Syria. In the first five years of the war, which began in 2011, an estimated 400,000 Syrians were killed, according to the UN Envoy for Syria.
Bashar al-Assad has ruled Syria as president since July 2000. His father, Hafez al-Assad, ruled Syria from 1970-2000.
As of April 2018, more than 5.6 million Syrians have fled the country, according to the UN HIgh Commissioner for Refugees, and more than 6 million people are displaced internally.
When the civil war began in 2011, there were four main factions of fighting groups throughout the country: Kurdish forces, ISIS, other opposition (such as Jaish al Fateh, an alliance between the Nusra Front and Ahrar-al-Sham) and the Assad regime. But as ISIS loses control of most of its territory, combatants are now freer to attack each other.
March 2011 - Violence flares in Daraa after a group of teens and children are arrested for writing political graffiti. Dozens of people are killed when security forces crack down on demonstrations.
March 24, 2011 - In response to continuing protests, the Syrian government announces several plans to appease citizens. State employees will receive an immediate salary increase. The government also plans to study lifting Syria's long standing emergency law and the licensing of new political parties.
March 30, 2011 - Assad addresses the nation in a 45-minute televised speech. He acknowledges that the government has not met the people's needs, but he does not offer any concrete changes. The state of emergency remains in effect.
April 21, 2011 - Assad lifts the country's 48-year-old state of emergency. He also abolishes the Higher State Security Court and issues a decree "regulating the right to peaceful protest, as one of the basic human rights guaranteed by the Syrian Constitution."
May 18, 2011 - The United States imposes sanctions against Assad and six other senior Syrian officials. The Treasury Department details the sanctions by saying, "As a result of this action, any property in the United States or in the possession or control of US persons in which the individuals listed in the Annex have an interest is blocked, and US persons are generally prohibited from engaging in transactions with them."
August 18, 2011 - The United States imposes new economic sanctions on Syria, freezing Syrian government assets in the US, barring Americans from making new investments in the country and prohibiting any US transactions relating to Syrian petroleum products, among other things.
September 2, 2011 - The European Union bans the import of Syrian oil.
September 23, 2011 - The EU imposes additional sanctions against Syria, due to "the continuing brutal campaign" by the government against its own people.
October 2, 2011 - A new alignment of Syrian opposition groups establishes the Syrian National Council, a framework through which to end Assad's government and establish a democratic system.
October 4, 2011 - Russia and China veto a UN Security Council resolution that would call for an immediate halt to the crackdown in Syria against opponents of Assad. Nine of the 15-member council countries, including the United States, voted in favor of adopting the resolution.
November 12, 2011 - The Arab League suspends Syria's membership, effective November 16, 2011.
November 27, 2011 - Foreign ministers from 19 Arab League countries vote to impose economic sanctions against the Syrian regime for its part in a bloody crackdown on civilian demonstrators.
November 30, 2011 - Turkey announces a series of measures, including financial sanctions, against Syria.
December 19, 2011 - Syria signs an Arab League proposal aimed at ending violence between government forces and protesters.
January 28, 2012 - The Arab League suspends its mission in Syria as violence there continues.
February 2, 2012 - A UN Security Council meeting ends with no agreement on a draft resolution intended to pressure Syria to end its crackdown on anti-government demonstrators.
February 4, 2012 - A UN Security Council resolution condemning Syria is not adopted after Russia and China vote against it.
February 6, 2012 - The United States closes its embassy in Damascus and recalls its diplomats.
February 7, 2012 - The Gulf Cooperation Council announces its member states are pulling their ambassadors from Damascus and expelling the Syrian ambassadors in their countries.
February 16, 2012 - The United Nations General Assembly passes a nonbinding resolution endorsing the Arab League plan for Assad to step down. The vote was 137 in favor and 12 against, with 17 abstentions.
February 26, 2012 - Syrians vote on a constitutional referendum in polling centers across the country. Almost 90% of voters approve the changes to the constitution, which include the possibility of a multi-party system.
March 13, 2012 - Kofi Annan, the UN special envoy to Syria, meets in Turkey with government officials and Syrian opposition members. In a visit to Syria over the weekend, he calls for a ceasefire, the release of detainees and allowing unfettered access to relief agencies to deliver much-needed aid.
March 15, 2012 - The Gulf Cooperation Council announces that the six member countries will close their Syrian embassies and calls on the international community "to stop what is going on in Syria."
March 27, 2012 - The Syrian government accepts Annan's plan to end violence. The proposal seeks to stop the violence, give access to humanitarian agencies, release detainees and start a political dialogue to address the concerns of the Syrian people.
April 1, 2012 - At a conference in Istanbul, the international group Friends of the Syrian People formally recognizes the Syrian National Council as a legitimate representative of the Syrian people.
July 30, 2012 - The Syrian Charge d'Affaires in London, Khaled al-Ayoubi, resigns, stating he is "no longer willing to represent a regime that has committed such violent and oppressive acts against its own people."
August 2, 2012 - UN Secretary General Ban Ki-moon announces that Annan will not renew his mandate when it expires at the end of August.
August 6, 2012 - Syrian Prime Minister Riyad Hijab's resignation from office and defection from Assad's regime is read on Al Jazeera by his spokesman Muhammad el-Etri. Hijab and his family are said to have left Syria overnight, arriving in Jordan. Hijab is the highest-profile official to defect.
August 9, 2012 - Syrian television reports that Assad has appointed Health Minister Wael al-Halki as the new prime minister.
October 3, 2012 - Five people are killed by Syrian shelling in the Turkish border town of Akcakale. In response, Turkey fires on Syrian targets and its parliament authorizes a resolution giving the government permission to deploy its soldiers to foreign countries.
November 11, 2012 - Israel fires warning shots toward Syria after a mortar shell hits an Israeli military post. It is the first time Israel has fired on Syria across the Golan Heights since the 1973 Yom Kippur War.
November 11, 2012 - Syrian opposition factions formally agree to unite as the National Coalition for Syrian Revolutionary and Opposition Forces.
November 13, 2012 - Sheikh Ahmed Moaz al-Khatib is elected leader of the Syrian opposition collective, the National Coalition for Syrian Revolutionary and Opposition Forces.
January 6, 2013 - Assad announces he will not step down and that his vision of Syria's future includes a new constitution and an end to support for the opposition. The opposition refuses to work with Assad's government.
March 19, 2013 - The National Coalition for Syrian Revolutionary and Opposition Forces elects Ghassan Hitto as its prime minister. Though born in Damascus, Hitto has spent much of his life in the United States, and holds dual US and Syrian citizenship.
May 27, 2013 - EU nations end the arms embargo against the Syrian rebels.
May 27, 2013 - US Senator John McCain visits rebels in Syria. It is reported that he is the highest ranking US official to visit since the beginning of the war.
June 13, 2013 - US President Barack Obama says that Syria has crossed a "red line" with its use of chemical weapons against rebels. His administration indicates that it will be stepping up its support of the rebels, who have been calling for the United States and others to provide arms needed to battle Assad's forces.
July 6, 2013 - Ahmad Assi Jarba is elected the new leader of the Syrian National Coalition.
August 18, 2013 - A team of UN weapons inspectors arrives in Syria to begin an investigation into whether chemical weapons have been used during the civil war.
August 22, 2013 - The United Nations and the United States call for an immediate investigation of Syrian activists' claims that the Assad government used chemical weapons in an attack on civilians on August 21. Anti-regime activist groups in Syria say more than 1,300 people were killed in the attack outside Damascus, many of them women and children.
August 24, 2013 - Medical charity Doctors Without Borders announces that three hospitals near Damascus treated more than 3,000 patients suffering "neurotoxic symptoms" on August 21. Reportedly, 355 of the patients died.
August 26, 2013 - UN inspectors reach the site of a reported chemical attack in Moadamiyet al-Sham, near Damascus. En route to the site, the team's convoy is hit by sniper fire. No one is injured.
August 29, 2013 - The UK's Parliament votes against any military action in Syria.
August 30, 2013 - US Secretary of State John Kerry says that US intelligence information has found that 1,429 people were killed in last week's chemical weapons attack in Syria, including at least 426 children.
September 9, 2013 - Syria agrees to a Russian proposal to give up control of its chemical weapons.
September 10, 2013 - In a speech, Obama says he will not "put American boots on the ground in Syria," but does not rule out other military options.
September 14, 2013 - The United States and Russia agree to a plan to eliminate chemical weapons in Syria.
September 16, 2013 - The United Nations releases a report from chemical weapons inspectors who investigated the August 21 incident. Inspectors say there is "clear and convincing evidence" that sarin was used.
September 20, 2013 - Syria releases an initial report on its chemical weapons program.
September 27, 2013 - The UN Security Council passes a resolution requiring Syria to eliminate its arsenal of chemical weapons. Assad says he will abide by the resolution.
September 30, 2013 - At the UN General Assembly in New York, Syrian Foreign Minister Walid al-Moualem says that Syria is not engaged in a civil war, but a war on terror.
October 6, 2013 - Syria begins dismantling its chemical weapons program, including the destruction of missile warheads and aerial bombs.
October 31, 2013 - The Organization for the Prohibition of Chemical Weapons announces that Syria has destroyed all its declared chemical weapons production facilities.
November 25, 2013 - The United Nations announces that starting January 22 in Geneva, Switzerland, the Syrian government and an unknown number of opposition groups will meet at a "Geneva II" conference meant to broker an end to the Syrian civil war.
December 2, 2013 - UN High Commissioner for Human Rights Navi Pillay says that a UN fact-finding team has found "massive evidence" that the highest levels of the Syrian government are responsible for war crimes.
January 20, 2014 - The Syria National Coalition announces it won't participate in the Geneva II talks unless the United Nations rescinds its surprise invitation to Iran or Iran agrees to certain conditions. The United Nations later rescinds Iran's invitation.
February 13, 2014 - The Organization for the Prohibition of Chemical Weapons tells CNN that Syria has shipped out 11% of its chemical weapons stockpile, falling far short of the February 5 deadline to have all such arms removed from the country.
June 3, 2014 - Assad is re-elected, reportedly receiving 88.7% of the vote in the country's first election since civil war broke out in 2011.
September 22-23, 2014 - The United States and allies launch airstrikes against ISIS targets in Syria, focusing on the city of Raqqa.
September 14-15, 2015 - A Pentagon spokesperson says the Russian military appears to be attempting to set up a forward operating base in western Syria, in the area around the port city of Latakia. Russian President Vladimir Putin says that Russia is supporting the Syrian government in its fight against ISIS.
October 30, 2015 - US White House spokesman Josh Earnest says that the US will be deploying "less than 50" Special Operations forces, who will be sent to Kurdish-controlled territory in northern Syria. The American troops will help local Kurdish and Arab forces fighting ISIS with logistics and are planning to bolster their efforts.
February 26, 2016 - A temporary cessation of hostilities goes into effect. The truce calls for the Syrian regime and rebels to give relief organizations access to disputed territories so they can assist civilians.
March 15, 2016 - Russia starts withdrawing its forces from Syria. A spokeswoman for Assad tells CNN that the Russian campaign is winding down after achieving its goal of helping Syrian troops take back territory claimed by terrorists.
September 15, 2016 - At least 23 people, including nine children, are killed during airstrikes in Syria, with the United States and Russia accusing each other of violating the ceasefire in effect since September 12.
September 17, 2016 - US-led coalition airstrikes near Deir Ezzor Airport intended to target ISIS instead kill 62 Syrian soldiers.
September 20, 2016 - An aid convoy and warehouse of the Syrian Arab Red Crescent are bombed; no one claims responsibility. The strike prompts the UN to halt aid operations in Syria.
September 23-25, 2016 - About 200 airstrikes hit Aleppo during the weekend, with one activist telling CNN it is a level of bombing they have not seen before.
December 13, 2016 - As government forces take control of most of Aleppo from rebel groups, Turkey and Russia broker a ceasefire for eastern Aleppo so that civilians can be evacuated. The UN Security Council holds an emergency session amid reports of mounting civilian deaths and extrajudicial killings. The ceasefire collapses less than a day after it is implemented.
December 22, 2016 - Syria's state-run media announces government forces have taken full control of Aleppo, ending more than four years of rebel rule there.
April 4, 2017 - Dozens of civilians are reportedly killed in a suspected chemical attack in the rebel-held town of Khan Sheikhoun. The Russian Defense Ministry claims that gas was released when Syrian forces bombed a chemical munitions depot operated by terrorists. Activists, however, say that Syrians carried out a targeted chemical attack.
April 6, 2017 - The United States launches a military strike on a Syrian government airbase in response to the chemical weapon attack on civilians. On US President Donald Trump's orders, US warships launch 59 Tomahawk cruise missiles at the airbase which was home to the warplanes that carried out the chemical attacks.
July 7, 2017 - Trump and Putin reach an agreement on curbing violence in southwest Syria during their meeting at the G20 in Hamburg, Germany. The ceasefire will take effect in the de-escalation zone beginning at noon Damascus time on July 9.
October 17, 2017 - ISIS loses control of its self-declared capital, Raqqa. US-backed forces fighting in Raqqa say "major military operations" have ended, though there are still pockets of resistance in the city.
October 26, 2017 - A joint report from the United Nations and international chemical weapons inspectors finds that the Assad regime was responsible for the April 2017 sarin attack that killed more than 80 people. Syria has repeatedly denied it had anything to do with the attack and also denies it has any chemical weapons.
February 24, 2018 - The UN Security Council unanimously approves a 30-day ceasefire resolution in Syria, though it is unclear when the ceasefire is meant to start, or how it will be enforced.
February 27, 2018 - Within minutes of when a five-hour "humanitarian pause" ordered by Putin -- from 9 a.m. to 2 p.m. -- is meant to start, activists on the ground report shelling and artillery fire from pro-regime positions, killing at least one person in the rebel-held enclave of Eastern Ghouta.
March 6, 2018 - More than 1,000 children have been killed or injured this year across Syria, UNICEF regional communications chief Juliette Touma tells CNN. In all, 342 children were killed and 803 were injured in Syria in the first two months of 2018, Touma says, citing multiple sources.
April 7, 2018 - Helicopters drop barrel bombs filled with toxic gas on the last rebel-held town in Eastern Ghouta, activist groups say. The World Health Organization later says that as many as 500 people may have been affected by the attack.
April 13, 2018 - The United States, France and the United Kingdom launch airstrikes on Syria in response to the chemical weapons attack in Eastern Ghouta a week earlier.