April 18, 2010 -- U.S. and Iraqi forces kill two top leaders of al-Qaida's branch in Iraq. Abu Bakr al-Baghdadi becomes the terror group's new leader.
March 2011 -- Syrian conflict erupts following an uprising against President Bashar Assad that turns into an insurgency and eventually civil war, attracting scores of al-Qaida militants from Iraq.
April 2013 -- Al-Baghdadi announces his group has taken over the Nusra Front, the al-Qaida affiliate in Syria. The combined group is variously called the Islamic State of Iraq and the Levant, or ISIL, and the Islamic State of Iraq and Syria, or ISIS; eventually it is known as the Islamic State group. Nusra denies the takeover, sparking infighting that continues to this day.
March 5, 2013 -- Rebel factions including the Nusra Front take over Raqqa, a provincial capital in north-central Syria, the largest city to be wrested from Assad's government by rebels.
May 2013 -- IS fighters burst on stage and show their strength by shooting three peoples in a main square in Raqqa. The group begins seizing towns and villages across northern Syria.
January 2014 -- Al-Baghdadi's forces sweep into Fallujah and parts of Ramadi in Iraq's Anbar province, which Iraqi security forces had abandoned weeks earlier. In Syria, they seize full control of Raqqa after driving out rival groups. Infighting with rivals erupts across north and eastern Syria.
June 10, 2014 -- Al-Baghdadi's fighters take over Iraq's second-largest city of Mosul and, the next day, overrun more of Nineveh province, including the village of Eski Mosul. They soon sweep south, capturing Saddam Hussein's hometown of Tikrit and much of the Sunni heartland nearly to the outskirts of Baghdad as government forces melt away. When they threaten Shiite holy sites, Iraq's top Shiite cleric issues a call to arms against the group, bringing a flood of volunteers to militias.
June 29, 2014 -- The IS group declares the establishment of a "caliphate," or Islamic state, in territories it controls in Iraq and Syria, and al-Baghdadi is declared the caliph. The militants rename themselves the Islamic State group. To celebrate, the militants "unite" the territories by knocking down sand berms in the desert that mark the border between the two countries. They soon after seize several actual border crossings.
Aug. 8, 2014 -- The U.S. begins targeting the IS group with airstrikes, citing the humanitarian plight of Iraq's minorities, like the Yazidis.
Aug. 19, 2014 -- IS releases a video showing the beheading of American journalist James Foley in response to the U.S.-led airstrikes. This marks the first of many videos showing militants beheading foreign captives.
Jan. 23, 2015 -- Kurdish peshmerga fighters regain control of Eski Mosul and several neighboring towns, driving out IS fighters.
Jan. 26, 2015 -- Kurdish fighters take control of the Syrian border town of Kobani near Turkey after fighting IS militants for months. U.S.-led airstrikes helped turn the tide for the Kurds.
Feb. 3, 2015 -- IS releases a video of Jordanian pilot Muath al-Kaseasbeh being burned to death inside a cage.
April 1, 2015 -- Backed by U.S.-led coalition airstrikes, Iraqi forces retake the city of Tikrit, their biggest gain yet against the IS group.
May 17, 2015 -- Ramadi, the capital of Iraq's Anbar province, falls to IS as Iraqi forces abandon their weapons and armored vehicles to flee.
May 20, 2015 -- IS captures the historic desert city of Palmyra in central Syria after Assad's forces collapse and withdraw.
June 16, 2015 -- Kurds deal the Islamic State a major setback by capturing the Syrian town of Tal Abyad on the Turkish border, which had provided a strategic supply route to IS.