November 12, 2015 9:19 AM EST

Some 100,000 flights take off and land around the world each day, every one of which relies upon suspension of the fear that rises when wheels lift off tarmac. So for a terrorist group, bringing down a civilian airliner is a marquee achievement. It’s something al-Qaeda has been trying and failing to do since Sept. 11. Now a consensus is forming that ISIS managed to pull it off in the month between the start of Russian strikes in Syria and Oct. 31, when Metrojet Flight 9268 exploded en route from Egypt to St. Petersburg.

Its destruction, killing all 224 on board, carries implications for air travelers well beyond the thousands of Russians stranded in the Egyptian resort of Sharm el-Sheikh, waiting for flights home on planes carrying no luggage in their holds (where a bomb may have been slipped onto the doomed jet). U.S. Homeland Security tightened surveillance at some foreign airports. Airport workers everywhere–and just about anyone who can get near a plane–can expect even more scrutiny as the holiday travel season kicks off.

But will it be enough, given what the apparent Sinai attack says about ISIS? It stands as the world’s ultimate terrorist group, operating on levels that no previous extremist organization has ever reached. It exists simultaneously as a military force, a political movement and a terrorist franchise capable of spectacular attacks.

The military piece clearly remains the group’s primary preoccupation, and for good reason. Actually holding territory–the swath of Iraq and Syria it declared a caliphate–is what first distinguished the group and what still animates its following. When volunteers in Western countries offered to do whatever ISIS wants–launch an attack where they live or hop a flight toward Syria–they were generally directed to join the fight in the Middle East. What attacks went forward in the West have been thrashing, lone-wolf efforts.

At the same time, ISIS has also taken a page from a lesser-known Islamist group: Hizb ut-Tahrir, or Party of Liberation. Hizb has been around for decades and has always been careful to refrain from advocating violence. It is a political, even bookish group with a utopian vision far removed from the bloody theology of ISIS–except for how that vision is supposed to come into being. Hizb advocates secretly placing followers in crucial positions throughout a secular state, then, when the time is ripe, seizing power through a coup.

It’s never actually worked–there have been only three conspiracies alleged in the group’s 63-year history, and all failed. Yet in the past four months, ISIS has inspired at least two: On Aug. 2, the United Arab Emirates announced that 41 people would go on trial for “trying to seize power and establish a caliphate.” Then on Oct. 28, Ethiopia arrested members of an ISIS cell allegedly planning to do the same. In May, in the Central Asian nation of Tajikistan, the U.S.-trained commander of the government’s special forces showed up in an ISIS video–not being beheaded, but as a fighter. He’d turned.

ISIS is now the thing al-Qaeda never managed to become: a movement that inspires volunteers. The source of that inspiration likely depends on the individual. Many may hark to the group’s dynamism–it’s actually fighting, not drifting, as many Muslims say their global community has been doing since the last acknowledged caliphate was abolished at the close of World War I. When he established his caliphate in June 2014, ISIS leader Abu Bakr al-Baghdadi’s call to arms appealed to the sense of aggrievement that has bedeviled Muslim politics since: “So rush O Muslims and gather around your [caliphate], so that you may return as you once were for ages, kings of the earth and knights of war. Come so that you may be honored and esteemed, living as masters with dignity.”

It’s hard to know in whose breast those words will swell. But investigators are looking closely at the people who worked at Sharm el-Sheikh airport. It may have been infiltrated by Sunni extremists from the region who last year named Sinai a “province” of the ISIS caliphate, while retaining operational autonomy. (Investigators say ISIS headquarters in the Syrian town of Raqqa apparently did not know in advance about any plot to down the plane.) If ISIS’s leaders turn out to like the idea, the world will be that much harder to keep safe.

This appears in the November 23, 2015 issue of TIME.

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