Known for its ruthless tactics and suicide bombers, ISIL has already controlled the Iraqi city of Fallujah for five months, and is also arguably the most capable force fighting President Bashar al-Assad inside Syria.
Its takeover of Mosul yesterday prompted the United States to voice deep concern about the "extremely serious" situation and warn the jihadist Sunni group poses "a threat to the entire region".
UN chief Ban Ki-moon's spokesman said he was "gravely concerned by the serious deteriorating of the security situation in Mosul".
Western governments fear it could eventually emulate Al-Qaeda and strike overseas, but their biggest worry for now is likely the eventual return home of foreign fighters attracted by ISIL and Baghdadi.
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Among them are men like Mehdi Nemmouche, a 29-year-old Frenchman who allegedly carried out a deadly shooting on a Jewish museum in Belgium last month after spending a year fighting with ISIL in Syria.
And ISIL appears to have the greatest appeal, with King's College London Professor Peter Neumann estimating around 80 percent of Western fighters in Syria have joined the group.
Unlike other groups fighting Assad, ISIL is seen working towards an ideal Islamic emirate that straddles Syria and Iraq. And compared with Al-Qaeda's franchise in Syria, Al-Nusra Front, it has lower entry barriers.
ISIL has also sought to appeal to non-Arabs, recently publishing two English-language magazines, having already released videos in English, or with English subtitles.