The philosopher Friedrich Nietzsche went on holiday more than a century and a quarter ago, and wrote a little pamphlet, Twilight of the Gods, that contained a timeless line: “From life’s school of war: what does not kill me makes me stronger.”

You could say the same about our current conflict, against the so-called Islamic State. After a year of limited airstrikes and half-hearted intervention, the American military has not killed ISIS. And while the terror army’s military capability may have been degraded, their reputation has only grown. After all, they’ve taken on the world’s only superpower—and are still standing.

A top U.S. intelligence official acknowledged only a week ago that they have produced a de facto “caliphate”—a state run according to strict Islamic law—just by holding territory in Syria and expanding to Iraq and beyond.

While ISIS lost an estimated 2,000 fighters over the past year, the flow of new recruits from outside the country helps them muster between 20,000 and 30,000 fighters, according to a U.S. intelligence official and a U.S. defense official.

Both spoke anonymously to discuss the story behind the figures, which were released to the press in July.

And while the numbers hold steady in Syria and Iraq, they are growing elsewhere.

“They are looking for opportunities where there are ungoverned spaces,” Special Operations commander General Joseph Votel said at the recent Aspen Security Forum in Colorado. “When pressure is applied on them in the Middle East…the bulge just comes out someplace else,” from Libya to Nigeria to Afghanistan, he said.

It’s a grim coda for Obama, who hoped to see militancy diminish after the killing of Osama bin Laden and the crippling of his al Qaeda enterprise in Afghanistan and Pakistan. That, combined with drawing down U.S. troops in Iraq and Afghanistan, was supposed to rob militant groups of the “foreign invaders” recruiting tool.

But that’s not how things turned out. By taking over large swaths of Iraq and Syria—and by beheading U.S. hostages—ISIS goaded America and its allies into attacking them. In the worldwide jihadist community, those strikes were considered a badge of honor—especially because ISIS was able to withstand the blows.

Ordinarily, the goal of an insurgent movement is simply to survive. ISIS did much more than that. It took over major cities like Ramadi and Fallujah in Iraq—losses for which the U.S. was often blamed. It forced the U.S. into uneasy alliance with Shi’ite militias—which only nudged the local Sunni populations into ISIS’s corner. And over the past year, ISIS has made inroads throughout Africa and the Middle East.

“Our greatest concern is ISIL and their declaration, and in fact, the existence of their caliphate as they are extending franchises, and sub-franchises, the most developed of which is in Libya,” said Director of National Intelligence James Clapper, speaking at the Aspen forum and using the government’s preferred acronym for the terror group.

Al Qaeda in Yemen is still most likely to bring an airliner down, but ISIS is drawing more followers, he said.

“There’s a competition for global jihad between these two groups, and ISIL is pulling ahead,” said just-retired Pentagon intelligence chief Mike Vickers in an interview.

“Over time, particularly if they maintain a sanctuary, then they could move up higher in the threat chain as well,” he said.

When asked during his Aspen panel if the group had been created by the U.S. invasion of Iraq, Vickers answered, “Create it? Technically, no, al-Qaeda in Iraq was formed before the U.S. invasion in 2002 as [founder] Abu Musab al Zarqawi left Afghanistan after the U.S. invasion [there] and made his way into Iraq.”

“Did it intensify the growth of al-Qaeda in Iraq, ISIL’s predecessor organization?… You bet it did,” he added.

Yet military officials in charge of the ISIS offensive continue to strike a hopeful tone.

“They are defending more than they are on the offensive,” said Marine Corps Brigadier General Kevin Killea, chief of staff for the ISIS task force.

“Their attacks are smaller. They’re more focused and less enduring,” Killea said, speaking to reporters Friday from an undisclosed location in Southeast Asia.

But when it comes to potential attacks in the states, it’s the number of small attacks ISIS is able to inspire that makes the group more dangerous than al Qaeda, at least in the near term.

“It’s a matter of scale,” said Nicholas Rasmussen, the director of the National Counterterrorist Center, to The Daily Beast. “The tactics used in ISIL-inspired attacks in the West that we have seen so far have been limited to firearms and small explosives, like IEDs,” or improvised explosive devices. That doesn’t minimize the tragedy or sense of loss, he said.

ISIS’s ranks in Syria and Iraq include “hundreds” of Westerners, and a “few dozen” Americans—a slightly higher estimate than before, according to the intelligence estimate based on figures gathered from January to May of this year.

The intelligence official attributed the drop in ISIS fighters to “better reporting by foreign partners, coalition strikes, [and] foreign fighter flows.”

Overall, more than 25,000 foreign fighters from more than 100 countries have traveled to Syria, and least 4,500 are Westerners, including 250 “U.S. persons” who have traveled to Syria or Iraq, or tried to get there.

“This includes those who are there now, those who have been stopped from traveling, and those who have returned,” the official said Tuesday, speaking on condition of anonymity to discuss the intelligence analysis.

More than 20 of the U.S. travelers have died, he added.

The Obama strategy to defeat ISIS relies largely on airpower, and special operations training and advising of Iraqi and Kurdish forces, as well as Syrian rebels. The program to train rebels is far behind, with fewer than 100 trained instead of the 5,000 the Americans hoped for. (As many as 20 of those rebels were kidnapped by al Qaeda’s franchise in Syria this week.) Worse, the rebels most willing to fight are pulling out of the program because the U.S. doesn’t want them attacking their main enemy: Syrian dictator Bashar al Assad.

“The numbers aren’t great, but I’m confident” in the program, special operations chief Votel said. “I don’t think there is any one strategy that we can apply that is immediately going to change this,” he said of the crisis.

He compared the fight against ISIS to Columbia’s five-decade-long battle to defeat the Revolutionary Armed Forces of Colombia (FARC), with the assistance of U.S. special operations and intelligence. Columbia has sometimes checked but never eliminated the group.

Daniel Glaser—the Treasury Department’s point man in the fight against ISIS—said it would take an unknowable amount of time to start cutting the terror army off from the economic resources that keep it afloat. He said there was up to $1 billion in cash in the banks in cities ISIS captured—money that will eventually run out, but a hefty startup fund to draw from.

“I wish I could put an ETA on it,” he said in an interview. “It takes time.”

The group pays some $360 million a year in salaries for its estimated 30,000 fighters at a rate of $1,000 a month, according to Glaser’s back-of-the-envelope calculations. ISIS is bringing in more than enough to keep that going even once it’s bank vault windfall runs out, through oil sales, extortion, kidnapping, and taxing those under its rule, plus the group takes a cut from the Iraqi government salaries that have continued to flow into areas under its control.

The group’s prodigious power to recruit through thousands of social media accounts is what makes it so dangerous, prompting White House Counterterrorism Advisor Lisa Monaco to call for Twitter and other social media sites to take ISIS accounts down. Civil liberty and privacy activists like Democratic Senator Ron Wyden have argued against such actions. (On Tuesday, the Oregon lawmaker placed a hold on a bill “that would require companies like Twitter and Facebook to make judgments about when users’ speech constitutes ‘terrorist activity,’” according to a statement from his office.)

Whereas al Qaeda used to require a would-be operative to visit Pakistan or Afghanistan for vetting and training in sophisticated bomb-making techniques or similar, ISIS vets its operatives’ loyalty by challenging a U.S.-based follower to go kill someone, or sends an American in Syria to the front line, according to a former U.S. intelligence official at the conference. If the operative dies in the process, it still produces a bonanza of publicity for the group.

But those one-off attacks don’t compare to taking down an airliner carrying hundreds, as al-Qaeda’s Yemeni offshoot still seeks to do.

“I am still concerned about al Qaeda’s ability to develop well planned, large-scale attacks…so I worry about them both,” NCTC chief Rasmussen added.

Al Qaeda in the Arabian Peninsula continues to focus on the West, according to multiple officials at the conference, even as it battles as one of many factions vying for control in Yemen’s civil war.

U.S. forces had to withdraw early this year, and Yemeni counterterrorist forces are too busy trying to restore the former regime to power to be of much help, according to a U.S. intelligence official who has been following the conflict. The official was not authorized to share this analysis publicly.

“We’ve always felt that AQAP was capable of compartmenting its external operations efforts to protect and shield those operations from other activities like conventional warfare against the Yemeni state,” the official said.

“The chaos in Yemen has actually increased the size of the potential safe haven to plot and plan operations," he said, adding that a core set of AQAP members continue to focus on attacking the West, including underwear bomb maker Ibrahim al Asiri. One of Asiri’s earlier creations tried but failed to bring down a U.S.-bound airliner in 2009.

“I think they have the ability to regenerate or reinvent themselves,” said special operations chief Votel. “They might be down, but they’re not out… The moment we take our eye off them, we’re vulnerable.”

Could ISIS and al Qaeda merge in a nightmare team that combines followers and forces? They have cooperated on the small scale, the intelligence official and an administration official said. In Libya, when a U.S. drone tried to take out al Qaeda leader Mokhtar Belmokhtar, there were also ISIS operatives at the safe house for some sort of meeting, the administration official said.

But so far, there’s no sign of a “strategic realignment or common cause,” the intelligence official said.

“The longer the conflict goes on, and al Qaeda is reduced in strength and numbers, the less likely that ISIL will seek accommodation with al Qaeda,” the official said, because al Qaeda simply won’t have anything to bring to the table in terms of manpower or prestige.

The bombing that killed at least nine in Mogadishu last weekend during Obama’s trip to the region was claimed by al Shabaab, the Somali-based group that is reportedly considering pledging loyalty to ISIS, overturning its 2012 pledge to al Qaeda.

In that one example lies both the difficulty U.S. counterterrorist forces face, and what they hope will be the solution for the future, according to Votel, whose last post was to lead the elite counterterrorist force, the Joint Special Operations Command (JSOC).

Al Shabaab released a statement that the hotel attack was aimed at Westerners, in retaliation for the Western-backed offensive by African Union peacekeeping troops.

It’s those troops, assisted by the JSOC and U.S. intelligence, that are bearing the brunt of the fight—and that’s what the Obama administration wants to build in Iraq and Syria, throughout Africa and beyond: a network of local partners or proxies to fight in lieu of sending large numbers of U.S. troops, ever again.

What does the increased threat mean for the American public? Newly minted TSA chief Peter Neffenger told The Daily Beast that he wants travelers to be more vigilant, everywhere, because the potential enemy is going to look pretty normal because it really may be the guy next door.

“When we say see something, say something, it doesn’t mean wake up every once in a while and look around,” he said. “The safety of the system depends on people within that system.”

He and his team are puzzling over how to help Americans go beyond “see something, say something,” without turning them into vigilantes who are suspicious of anyone who looks, say, foreign or Muslim.

“Maybe somebody dressed inappropriately for the weather or somebody acting inappropriately,” he said. “I don’t want the traveling public member to start taking action. Just tap somebody from law enforcement on the shoulder and allow them to examine whether there’s a problem.”

Unfortunately, he and other administration officials fear ISIS may give Americans a lot of opportunity to practice.

This story was corrected to reflect that Somalia-based militant group Al-Shabaab has not yet officially pledged loyalty or “bayat” to ISIS.