In early June, ISIS conquered the northern Iraqi cities of Mosul and Tikrit, and has more recently been threatening to overtake the capital, Baghdad.
In response, Iraqi Prime Minister Nouri al-Maliki has called on his countrymen to take up arms against the group, while also beseeching the United States to provide military assistance, including air strikes.
Many analysts believe that the future of Iraq lies in the balance.
Here's a look at the group's genesis and what ISIS hopes to achieve.
What is it and what do members want?
ISIS is a group of Sunni jihadists led by 43-year-old Abu Bakr al-Baghdadi, the nom de guerre of Awwad Ibrahim al-Badri al-Samarri.
Baghdadi fought in an armed rebellion in western Iraq after the U.S. invasion in 2003, and was captured by U.S. forces in 2006. Upon his release towards the end of the decade, Baghdadi rejoined al-Qaeda affiliates in Iraq.
ISIS seeks to create an Islamic state in eastern Syria and northern Iraq based on sharia law. Thus far, the group has captured territory from the fringes of Aleppo in eastern Syria to Falluja in southern Iraq to Mosul in the north.
There has been much conjecture about the size of this group. According to London's Guardian newspaper, ISIS was thought to have around 6,000 members prior to seizing Mosul and Tikrit. But the conquest of key Iraqi cities has undoubtedly inspired many jihadis, both in and outside Iraq, to join.
How did the group emerge?
The roots of ISIS are in Iraq, but the group was greatly influenced by its experiences fighting in Syria, says Kamran Bokhari, vice-president of Middle Eastern and South Asian affairs for the geopolitical intelligence firm Stratfor.
ISIS's etymology can be traced to al-Qaeda in Iraq, which arose in response to the U.S. invasion in 2003. The group was active in the insurgency in Iraq, but when civil war broke out along sectarian lines in Syria in 2011, many ISIS members crossed the border to fight with other al-Qaeda-affiliated groups against the Shia Alawite regime of Syria's President Bashar al-Assad.
In 2013, ISIS came into a conflict with fellow jihadists, says Bokhari.
ISIS "hit a dead end in Syria, because it was fighting on two fronts – both the Assad regime and its allies as well as rival rebel groups that were relatively moderate," says Bokhari. As a result, "It decided it was in its interest to go back into Iraq."
Is it a part of al-Qaeda?
No. That's because while ISIS was fighting under the auspices of al-Qaeda in Syria, Baghdadi clashed with Ayman al-Zawahiri, the reclusive head of al-Qaeda, over strategy.
When the Syrian war broke out, Baghdadi sent an envoy to create the al-Nusra Front, which became the main Sunni jihadist group fighting the Shia regime of Assad. Zawahiri became angered, however, when Baghdadi tried to merge the al Nusra Front with ISIS, says Jabeur Fathally, a Middle East expert at the University of Ottawa.
ISIS ultimately split from al-Qaeda because Baghdadi's group had a different goal in mind: capturing swaths of territory to establish a caliphate, or Islamic state.
Differing goals also means differing enemies, says Fathally.
In October 2013, Zawahiri released a statement commanding al-Qaeda members "not to fight the deviant groups like the [Shia], the Ismailis, the Qadiyaniat, and deviant Sufis," and focus on "fighting the head of international infidelity," namely the U.S.
ISIS, on the other hand, is more focused on regional battles, especially against those who support the Shia-led Iraqi government under Nouri al-Maliki, says Benjamin Friedman, research fellow in defence and homeland security studies at the Cato Institute, a libertarian think-tank headquartered in Washington, D.C.
"I think it's unfortunate that [ISIS is] often referred to as an al-Qaeda group," Friedman says.
"Their ideology tends in that direction, but their actions so far, no matter how violent and terroristic, are of a group interested in local politics."
How is ISIS funded?
ISIS's funding is "a combination of outside private donations and domestically generated revenue in areas that they control," says Rex Brynen, a political science professor at Montreal's McGill University.
The group's biggest patrons are in the Gulf states, says Fathally.
"A large part of their financing comes from Saudi Arabia and Qatar, but it's not from official institutions, but from private wealthy people," he said.
The group is also enriching itself every time it conquers territory. Earlier this month, ISIS members reportedly looted $450-million from a bank in Mosul and helped themselves to military equipment left behind by members of the fleeing Iraqi army.
Brynen says ISIS also raises significant money through taxation and extortion.
Some of the territory it has captured, including the town of Raqqa in Syria, contains oil fields, which provides the group with another form of revenue. According to Stratfor's Bokhari, ISIS has sold oil to foreign buyers, including one of its stated enemies — the Syrian regime.
How much support does the ISIS get from regular Iraqis?
While much of the world has been surprised by the speed with which the ISIS has been able to take over Iraqi cities, "this could not happen without local support," says Henry Habib, professor emeritus of political science at Montreal's Concordia University.
He says that the group has been able to take advantage of Sunni outrage with the Shia-led government of Nouri al-Maliki.
"The failure of the Iraqi government in including the Sunnis and giving them positions in the government and so on and so forth has [had] a direct effect" on the ISIS's strategic success, Habib says.
One of the reasons they were able to take cities like Mosul and Tikrit is "because these are fundamentally Sunni cities that are tired of Maliki," says Michael O'Hanlon, director of foreign policy research at the Brookings Institute in Washington, D.C.
Cato's Friedman says that although the ISIS is getting most of the credit for its strategic gains, it has been abetted by a variety of smaller Sunni organizations, including tribal militias and Baathist fighters with links to the deposed regime of Iraqi dictator Saddam Hussein.
In a video posted to YouTube, ISIS spokesman Shaykh Muhammad Adnani said that the group "has not prevailed by numbers, nor equipment, nor weapons, nor wealth, rather it prevails by Allah’s bounty alone, through its creed.”