The Islamic State (IS) group seized swaths of land in Iraq and expanded its territory in Syria in a dramatic blitz in 2014, taking advantage of unrest in both countries. The militant group slaughtered civilians in its march to try to establish a radical caliphate, and has spawned a string of deadly attacks across Europe, the Middle East and the US.
In response, the US and a coalition of allies launched a sustained campaign of airstrikes in Iraq and Syria in 2014, and have been training, advising and supporting local forces in both countries. Recently, the US added Libya to its airstrike targets to root out extremists at the request of the Libyan government. While still a potent force, IS militants have lost much of the territory they overran. Meantime, they've stepped up attempts to inspire followers abroad to strike on their own, with some devastating results.
Hillary Clinton, who was secretary of State from 2009 to 2013, has described a three-part strategy that involves crushing IS on its home turf in the Middle East, disrupting their infrastructure on the ground and online, and protecting America and its allies. All are current elements of the Obama administration's strategy, so it's not clear what would change or if she would accelerate any portions of it. She's vowed, "We are not putting ground troops into Iraq ever again. And we're not putting ground troops into Syria. We're going to defeat ISIS without committing American ground troops."
Donald Trump promises to "bomb the hell" out of IS, and level the oil facilities it controls. He has provided no details, including whether he would increase US airstrikes or commit ground troops. And US airstrikes have already been doing precision bombing of oil facilities for some time.
Trump has also said he believes in enhanced interrogation techniques, which can include water-boarding and other types of torture that are against the law and that many experts argue are ineffective.
The IS group has specifically targeted the US and the West, using its networks, online communications and social media to attract foreign fighters to the front lines and followers in other countries to take up the fight overseas.
As the group comes under increasing pressure from the airstrikes and US-backed forces in Iraq and Syria, it has turned greater focus on inspiring lone-wolf attacks that are far more difficult to predict and prevent.
In San Bernardino, for example, investigators found that Syed Rizwan Farook and his wife pledged allegiance to the IS militants before the December attack that left 14 dead.
The group has also been linked as a possible inspiration, or claimed responsibility, for the November attacks in Paris; the subway and airport bombings in Brussels; the Orlando, Florida, nightclub shootings, the Bastille Day truck attack in Nice, France, the knife attack at a mall in Minnesota, and more. There are signs that accused New York bomber Ahmad Khan Rahami was radicalised abroad by Islamic extremists, though any ties to IS are tenuous.
President Barack Obama says IS militants have figured out that if they can persuade "a handful of people or even one person to carry out an attack on a subway, or at a parade or some other public venue, and kill scores of people as opposed to thousands of people, it still creates the kinds of fear and concern that elevates their profile."
The administration, however, has been criticised by some for not moving more aggressively and quickly to drive the group from its strongholds in Iraq and Syria. Some members of Congress have called for a stronger US military response. And officials have expressed frustration over the slow-moving effort to disrupt the militant group's online presence.