The Islamic State of Iraq enters the Syrian Civil War and begins by declaring a merger with the Al-Nusra Front under the name Islamic State of Iraq and ash-Sham

The Syrian civil war (Arabic: , romanized: al-arb al-ahlyah as-sryah) is an ongoing multi-sided civil war in Syria fought between the Syrian Arab Republic led by Syrian president Bashar al-Assad (supported by domestic and foreign allies) and various domestic and foreign forces that oppose both the Syrian government and each other, in varying combinations.Unrest in Syria began on 15 March 2011 as part of the wider 2011 Arab Spring protests out of discontent with the Syrian government, eventually escalating to an armed conflict after protests calling for Assad's removal were violently suppressed. The war is currently being fought by several factions, including the Syrian Armed Forces and its domestic and international allies, a loose alliance of mostly Sunni opposition rebel groups (such as the Free Syrian Army), Salafi jihadist groups (including al-Nusra Front and Tahrir al-Sham), the mixed Kurdish-Arab Syrian Democratic Forces (SDF), and the Islamic State of Iraq and the Levant (ISIL). The peak of the war was around 2015; violence in the country has since diminished, but the situation remains a crisis.A number of foreign countries, such as Iran, Russia, Turkey, and the United States, have either directly involved themselves in the conflict or provided support to one or another faction. Iran, Russia, and Hezbollah support the Syrian Arab Republic and the Syrian Armed Forces militarily, with Russia conducting airstrikes and other military operations since September 2015. The U.S.-led international coalition, established in 2014 with the declared purpose of countering ISIL, has conducted airstrikes primarily against ISIL as well as some against government and pro-government targets. They have also deployed special forces and artillery units to engage ISIL on the ground. Since 2015, the U.S. has supported the Autonomous Administration of North and East Syria and its armed wing, the Syrian Democratic Forces (SDF), materially, financially, and logistically. Turkish forces have fought the SDF, ISIL, and the Syrian government since 2016, but have also actively supported the Syrian opposition and currently occupy large swaths of northwestern Syria while engaging in significant ground combat. Between 2011 and 2017, fighting from the Syrian civil war spilled over into Lebanon as opponents and supporters of the Syrian government traveled to Lebanon to fight and attack each other on Lebanese soil, with ISIL and al-Nusra also engaging the Lebanese Army. Furthermore, while officially neutral, Israel has exchanged border fire and carried out repeated strikes against Hezbollah and Iranian forces, whose presence in southwestern Syria it views as a threat.International organizations have accused virtually all sides involved, including the Ba'athist Syrian government, ISIL, opposition rebel groups, Russia, Turkey, and the U.S.-led coalition of severe human rights violations and massacres. The conflict has caused a major refugee crisis, with millions fleeing mainly to neighboring countries Turkey, Lebanon and Jordan. Over the course of the war, a number of peace initiatives have been launched, including the March 2017 Geneva peace talks on Syria led by the United Nations, but fighting has continued.

The Islamic State of Iraq (ISI; Arabic: دولة العراق الإسلامية Dawlat al-ʿIrāq al-ʾIslāmiyyah), commonly referred to as al-Qaeda in Iraq (Arabic: القاعدة في العراق al-Qāʿidah fī al-ʿIrāq), was a militant Salafist jihadist group that aimed to establish an Islamic state in Sunni, Arab-majority areas of Iraq during the Iraq War and later in Syria during the Syrian Civil War.

Islamic State of Iraq traces its origins to Jama'at al-Tawhid wal-Jihad, which was formed by the Jordanian national Abu Musab al-Zarqawi in Jordan in 1999. Al-Zarqawi led the group, under numerous name changes, until his death in June 2006. Jama'at participated in the Iraqi insurgency (2003–2011) following the 2003 invasion of Iraq by Western forces, and on 17 October 2004 al-Zarqawi had pledged allegiance to Osama bin Laden's al-Qaeda network; and the group became known as Tanzim Qaidat al-Jihad fi Bilad al-Rafidayn (commonly known as al-Qaeda in Iraq). In January 2006, Tanzim and five other Iraqi insurgent groups formed the Mujahideen Shura Council, which on 15 October 2006 merged to form Islamic State of Iraq. At their height in 2006–2008, ISI had military units or strongholds in Mosul and in the governorates of Baghdad, Al Anbar and Diyala, and they claimed Baqubah as their capital. The area under its control decreased dramatically following the 2007 troop surge, during which dozens of ISI leaders were killed by coalition forces.

The new group continued to be commonly referred to as al-Qaeda in Iraq. Shortly after al-Zarqawi's death, al-Qaeda in Iraq named a new leader, Abu-Hamzah al-Muhajir, thought to be a pseudonym, which the US military named as Abu Ayyub al-Masri, an Egyptian militant based in Baghdad. Al-Masri and ISI leader Abu Omar al-Baghdadi were killed during a military operation on a safehouse on 18 April 2010. Abu Omar al-Baghdadi was succeeded as leader of ISI by Abu Bakr al-Baghdadi. On 14 May 2010, al-Masri was succeeded by Abu Suleiman al-Naser (also known as al-Nasser Lideen Illah Abu Suleiman), who was in turn killed some time in 2011. Following Suleiman's death, the position of "War Minister" was replaced by a Military Council composed of former regime military officers under the leadership of Haji Bakr.On 7 April 2013 Abu Bakr al-Baghdadi transformed ISI into the Islamic State of Iraq and the Levant (ISIL, ISIS, IS), which is still active today. Haji Bakr, whose name was Samir Abd Muhammad al-Khlifawi, was killed in January 2014, and was succeeded by Abu Abdulrahman al-Bilawi as head of the ISIL Military Council. Al-Bilawi was killed on 4 June 2014, and was reportedly succeeded by Abu Mohannad al-Sweidawi as leader of the ISIL Military Council. There were reports in November 2014 that al-Sweidawi had been killed in an Iraqi airstrike that reportedly also injured Abu Bakr al-Baghdadi. The Daily Beast reported that al-Sweidawi was succeeded by senior ISIL figure Abu Ali al-Anbari, who was in turn killed on 24 March 2016. Al-Anbari was considered the ISIL second-in-command in Syria and was viewed as a potential successor of ISIL's present leader Abu Bakr al-Baghdadi. The second-in-command in Iraq was Abu Muslim al-Turkmani, who was killed on 18 August 2015, and who was succeeded as the ISIL leader in Iraq by Abu Fatima al-Jaheishi.