PARIS—Now there’s a pause. There always is after the United States sets out to teach an explosive lesson to some entrepreneur of terror in the Middle East. Weeks pass, or months—even years after a bombing raid or cruise missile attack, before the retaliation comes. It's usually at a time and place where it’s not expected. Most often it is carried out by operatives who have no clear ties to their sponsors. We saw that pattern with Gaddafi, we saw it with Saddam, and we have seen it now for almost five decades with the Assad regime.

There’s an old adage that revenge is a dish best served cold, and nobody understands that better than the Assad family. Their dynasty has relied on spies and terror organizations to keep them in power since 1971. Its founder, Hafez Assad, was famous for his ruthless patience, like a serpent resting comfortably on its coils just waiting to strike. And now there are growing concerns among Western counterterror analysts that his son Bashar will respond to the American cruise missile attack ordered by U.S. President Donald Trump with a terror plot somewhere outside the borders of Syria.

Assad certainly has the means and the connections. In addition to his own intelligence services and clients, the coalition that sustains him includes Iran and its Lebanese extension Hezbollah, which is a seasoned militia, a political party, and one of the most well-organized and disciplined terror organizations on the planet. Some security analysts worry that Iranian agents will attack American forces in the region. Hundreds of U.S. personnel are now on the ground in Syria, thousands in Iraq. All are in harm's way.

But any link to Hezbollah and Iran probably would be too incriminating. The more effective play for Assad would be to encourage and perhaps facilitate a major attack, most likely in Europe, by the so-called Islamic State.

ISIS may be Assad’s putative enemy, but it has served his interests repeatedly, as Pulitzer Prize-winner Roy Gutman documented in a series of articles for The Daily Beast.

Assad first tried to ingratiate himself with Western leaders by portraying the national uprising against him in 2011 as a terrorist-led revolt. When that failed, he released jailed Islamic extremists who’d fought against U.S. troops in Iraq, then staged phony attacks on government facilities, which he blamed on terrorists. Far from fighting ISIS, Assad looked the other way when it set up a state-within-a-state with its capital in Raqqa, and then left it to the U.S. and others to try to take the battle to the Islamic extremists.

In reaction to the U.S. attack, Assad "could, at a minimum, keep for himself intelligence [about ISIS] that he has, or let Daesh [ISIS] people move out of Syria," says Claude Moniquet, a Belgian authority on Islamic State operations in Europe. "Or he could even use people that his service controls to organize things. This would be typical and 'good' Middle Eastern 'non-conventional warfared,' as we saw in Lebanon for 30 years."

Hafez and Bashar Assad used patience and terror to reduce that neighboring country to the condition of a vassal state, as Nadette De Visser and I have written in some detail. Among the attacks Hafez facilitated: two massive bombings of the American embassy in two different locations, the first of which virtually wiped out the CIA station, and the enormous suicide truck bomb that destroyed the U.S. Marine Battalion Landing Team barracks in 1983, killing 241 American service personnel.

In 2005, after Lebanon's billionaire Prime Minister Rafik Hariri fell out with Bashar Assad, Hariri was blown up along with 21 other people in the heart of Beirut. The Syrians claimed an obscure Sunni jihadist group carried out the suicide bombing. Assad said he was "100 percent innocent," of course. Then, over the years, four senior Syrian officials who might have implicated Assad directly were killed under mysterious circumstances. One "committed suicide" in his office under more than questionable circumstances. Another, Assad's brother-in-law, was blown up in a high-security compound, supposedly by the Syrian opposition. Another spy chief was reported killed fighting rebels in 2013. And one appears to have died after a savage beating by another intelligence chief in 2015. An international tribunal in the Hague has indicted five people in the Hariri murder case. In fact all of them are connected to the Shiite organization Hezbollah. None of them is in custody. They are protected from the court by Assad, but probably not from Assad himself.

So that's who we're dealing with. As Moniquet put it, all this is "typical" of the Assads: revenge by proxies, followed by assigning blame to supposed enemies, and then the murder of potential witnesses. Yet the emotional, instinctive reactions of the Trump administration are such that if there were a major ISIS attack in the United States, or even one in Europe, suddenly all those cross-hairs focused on Bashar Assad would shift again to ISIS. And Assad knows it.

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To the uninitiated, all of these intrigues within intrigues begin to sound like T. S. Eliot’s “wilderness of mirrors,” so I asked French scholar Gilles Kepel to walk me through it.

Kepel, who is on an ISIS hit list and has 24-hour police protection, has been researching and documenting the rise of jihadist movements since the assassination of Egyptian President Anwar Sadat in 1981. Fluent in Arabic and several other languages, he likes to explore the doublethink and triplethink of the Middle East’s terrorist organizations, and the sociological, economic, and political trends that have brought so many of the Muslims of Europe into a dangerous state of permanent confrontation with the societies around them. His recent book, Terror in France, was a best seller in his home country and is just about to be published in English in the United States.

I asked him about the impact on terrorism of the attacks in Syria.

“There were two attacks: the first was the Syrian attack,” he said: the use of the nerve agent sarin in the town of Khan Sheikhoun, killing scores of people including the “beautiful babies” whose deaths appear to have moved President Trump to action, and certainly moved much of the international community to outrage.

The Trump administration has strongly suggested there may have been Russian complicity in the attack, or, as Secretary of State Rex Tillerson put it, Moscow was “simply incompetent” to stop it.

“We don’t know exactly whether it was Assad acting on his own in order to twist the arm of his Russian patrons in some fashion, or whether this was something done in cooperation with the Russians,” said Kepel.

But the result was the second attack, by the United States on a Syrian airfield, which “suddenly boosted the image of Trump in the West. He was seen as erratic, suddenly he looked like someone with balls.”

“Now the Assad regime looks like a monster, and will be happy if it can deflect attention to ISIS,” said Kepel. He noted that after the horrendous November 13, 2015, attack in Paris by an ISIS commando that killed 130 people, French President François Hollande and other officials who had spent years lambasting Assad and calling for his removal suddenly decided that, yes, perhaps, “Assad was ‘less worse’ than ISIS.”

“So,” said Kepel, “we can expect that this is the right time for an ISIS strike, maybe with the help of some ‘obscure’ intelligence groups. Assad needs extra ISIS vilification.”

The Russians, too, might benefit from an ISIS attack in France. The country is in the final weeks of a presidential race that could decide the future not only of the nation, but of the European Union and NATO, two institutions Russian President Vladimir Putin would happily see destroyed. The most dynamic French contender, the far-right Marine Le Pen, wants to withdraw or downgrade French participation in both.

Parroting the Russian line, as she is wont to do, Le Pen said on Friday, “I’m a little surprised, because Trump has said many times he didn’t intend to make the United States the world’s policeman, and that’s exactly what he did yesterday. … Is it too much to ask that we see the results of an international investigation before carrying out this kind of strike?”

Closing in behind Le Pen in third place in the polls is former Prime Minister François Fillon, who is also famously friendly with Russian President Vladimir Putin, and one of whose books going into this campaign was Defeating Islamic Totalitarianism, which shows undisguised hostility toward many of France’s millions of Muslims.

Both candidates talk tough, and after a terror attack the public is receptive to such posturing. But ISIS strategists believe the recruiters of the so-called Islamic State would benefit if either Fillon or Le Pen were elected. As Kepel has pointed out many times, al Qaeda and ISIS want to alienate and isolate Europe’s Muslims from the culture around them, then call them to arms.

If there is good news, says Kepel, it is that the French secret services have caught up with the challenge posed by the jihadists, taking seriously the jihadists' hopes of inciting civil war, moving in on the networks, finding ways to break into their encrypted communications, and arresting people preemptively. The French intelligence services and police “have been tremendously efficient over the last several months,” Kepel told me.

The net result, according to Kepel, is that ISIS has stepped up its efforts to stage attacks in other European capitals, most notably with truck and car attacks in Berlin, and in London and Stockholm, although the question of direct coordination with ISIS in the latter two cases remains murky.

The fact that none of the many attacks plotted in France has been carried out successfully is extraordinary, said Kepel, but there’s also a strong feeling that the danger is growing.

Will Assad get his revenge for the Trump attack through ISIS? Will the Russians turn a blind eye? These questions are unanswerable, but they are being asked in many circles here in Europe, where the sense of vulnerability is high. “There is a lot of fear now,” said Kepel.

If there is a major new ISIS attack outside Syria, should Assad be held accountable? That may be the most crucial question of all.