In the mid-2000s, when the United States was heavily deployed in both Iraq and Afghanistan, the US administration was under repeated pressure domestically and internationally to declare when it would leave those countries. The administrations of George W Bush and later Barack Obama hesitated to do so, knowing that the moment a timeline was set, their adversaries would simply wait, biding their time until the clock ran down and the last soldiers went home.

The same calculation is being played out in the last Islamic State strongholds in eastern Syria, now that Donald Trump has declared ISIS “defeated” and announced that the estimated 2,000 American troops in the country are heading home.

The sudden end of the campaign against ISIS appears to have caught everyone, friend and foe, by surprise. If ISIS still poses a threat, it will simply lie low for several weeks until the US leaves and then re-emerge. If it has been defeated, it will no longer pose any threat. So, which is it?

The trouble is that claiming ISIS has been defeated is all a matter of definition. Specifically, the definition of two terms: “ISIS” and “defeated.”

In one sense, the battle against ISIS in Syria is coming to an end: This month saw the fall of Hajin, the last Syrian town held by the jihadis. But in another sense, ISIS exists more as an idea and less as a group, and that idea could yet re-emerge in different ways. It is possible for ISIS to exist in two states, simultaneously defeated and undefeated.

In one sense, it will never really make sense to speak of ISIS as defeated. There will always exist some forgotten corner of the globe or a dark crevice of the Internet in which recruits and recruiters will gather. The tangible legacy of ISIS’ tenure in Iraq and Syria – the testimonies of those involved, the documents of their time in charge, the photographs and gory and glorifying videos – will continue to exist, circulated on whichever communication method the supporters default to, waiting to be introduced to a new generation of the disaffected.

Groups can always rise again, and small groups suddenly become much more important. ISIS itself went through this evolution, rapidly overtaking al-Qaeda in terms of prominence and power. Al-Qaeda, too, in the years before September 11, 2001, was high but not foremost on the list of priorities of the US government. Bill Clinton, who served as president until a year before the 9/11 attacks, was relentlessly criticized for “missing” his opportunity to kill Osama bin Laden. The truth was that the US, like any government, had priorities.

There comes a time when political and military leaders accept that any military campaign faces diminishing returns, that continuing to supply resources to the fight against ISIS can no longer be justified politically

But in a different, more hard-headed sense, there comes a time when political and military leaders accept that any military campaign faces diminishing returns, that continuing to supply resources to the fight against ISIS can no longer be justified politically.

It certainly seems that Trump came to that conclusion after a phone call from Recep Tayyip Erdogan. The Turkish president’s question – “Why are you still there?” – focused Trump’s mind.

The consensus of analysts and of other governments is that ISIS is not quite defeated. One US estimate puts the number of fighters inside Syria at nearly 15,000. It is true that ISIS has lost more than 95% of the territory it held at its zenith, but many of its senior leaders, including Abu Bakr al-Baghdadi, remain alive and at large. The group’s formidable online presence continues.

But there’s a second part to being defeated. Because even if all those elements could be destroyed, ISIS could still re-emerge. The Kurdish-dominated Syrian Democratic Forces lack the ability to police large parts of Syria, and the Syrian regime may not be inclined to do so. Truly defeating ISIS would require rebuilding and policing the territories it formerly held, an open-ended commitment that Trump certainly does not want.

So, if the question is framed in narrow terms of the military defeat of ISIS such that it becomes difficult for it to regroup, then defeat is close, potentially measured in months. But then the second definition matters: What is ISIS?

The great power of ISIS always lay in an idea, expressed through the group. Yes, the group managed to establish a proto-state across Syria and Iraq. But its real strength – and its great danger – lay in its ability to motivate thousands of young men and women, from dozens of countries, to leave their settled lives and join this fledgling state. More than that, it managed to persuade them that they were doing so for an apocalyptic reason, for the chance to take part in the vanguard of a movement that would usher in a new global era.

Those ideas may seem laughable from the outside. But to thousands, they were astonishingly powerful. Understanding how ISIS managed to do that is crucial, because that idea could easily be expressed by a different, as yet unknown, group.

If ISIS were merely one group, it could be defeated. But if it is an idea, that idea will be extraordinarily difficult to defeat, and the battle against it will not be carried out in the eastern deserts of Syria, but in the hearts and minds of men and women across the world who might one day consider heeding the siren call of another ISIS.

Faisal Al Yafai