The 2011 uprising against President Bashar al-Assad spawned a violent, multisided conflict in Syria. As a result, opposition fighters claimed vast tracts of the country, allowing an array of armed groups space to gain strength.
As Syria’s civil war grew more complex, the remnants of a group once known as al-Qaeda in Iraq moved into Syria and became a powerful force among opposition groups. The Islamic State became known for its success in recruiting foreign fighters and for its zealous imposition of sharia law in the areas it captures.
In early 2014, Islamic State fighters took control of the city of Fallujah in western Iraq, revealing the fragility of Iraq’s security only two years after the departure of U.S. troops. Six months later, Islamic State militants streamed into northern Iraq and captured Mosul, Iraq’s second-largest city, taking the government in Baghdad and its western backers by surprise. The Islamic State’s advance was made possible by the collapse of Iraqi army units stationed around Mosul, raising questions about earlier U.S. efforts to build an effective military force in Iraq.
The arrival of the Islamic State proved calamitous for minorities in northern Iraq. The Islamic State executed Yazidis around the town of Sinjar; others faced harrowing conditions when they fled to nearby mountains. The United States launched its first airstrikes against the Islamic State, around Sinjar and in protection of the Kurdish capital, Irbil, in August 2014.
The Islamic State began to execute foreign hostages it held in Syria, including American journalists James Foley and Steven Sotloff. The killings galvanized public sentiment against the Islamic State. The next month, the Obama administration and its partners broadened their campaign of airstrikes to Syria.
President Obama authorized the return of substantial numbers of U.S. military personnel to Iraq for the first time since the 2011 withdrawal. American advisers were tasked with helping to rebuild a capable military force that can dislodge the Islamic State.
U.S. strikes helped partner forces in Iraq and Syria make limited gains against the Islamic State. In Syria, Kurdish forces took back control of Kobani, a city on the Turkish border. In Iraq, government forces pushed into the militant-held city of Tikrit.
Harsh conditions in areas under Islamic State control swelled the flows of migrants heading for Europe, exposing divisions among the 28-member European Union and heightening fears about militant attacks there.
The Obama administration gradually expanded its military role on the ground in a bid to accelerate progress against the Islamic State. While the White House insisted that U.S. troops would not take part in combat operations, troops helped local forces conduct dangerous operations. The first combat death occurred in October 2015, when American Special Operations forces accompanied Kurdish pershmerga troops on a hostage raid.
The reach of the Islamic State beyond Iraq and Syria became starkly clear when militants launched a series of coordinated attacks across Paris in November 2015, killing at least 130 people.
New attacks on the Brussels airport and metro system in March 2016 further illustrated the vulnerability of European nations to home-grown militant plots.
Although the United States has been shielded from the bulk of Europeans’ fears about migrant-related security threats, its own vulnerability was exposed in December 2015, when two California residents, voicing loyalty to the Islamic State, launched an attack on a gathering of municipal workers, killing 14 people and wounding at least 22 more. The attack, the deadliest terrorist attack on U.S. soil since Sept. 11, 2001, triggered an ongoing controversy about the limits of government access to personal data and the role of Muslim immigrants in the United States.