French Prime Minister Manuel Valls. (Lionel Bonaventure/AFP/Getty Images)

Jim Hoagland is a Post contributing editor and a visiting fellow at the Hoover Institution.

The European countries that have joined the U.S. air war on Islamic terrorists fight in a spirit of allied unity. But they are also driven by a mounting fear that their citizens who have fought for the jihadists in Syria and Iraq may soon return home to form an “enemy within” and mount a European 9/11.

Manuel Valls, the tough-minded French prime minister, sums up his nation’s existing security concerns this way: “We have no indication of a planned attack on French soil. But we have to be vigilant every day against the development of an enemy within.”

My conversation with the prime minister came a few days before terrorists in Algeria beheaded French tourist Herve Gourdel in a self-proclaimed act of solidarity with the Islamic State. Valls has long watched with mounting concern the growth of European involvement in what President Obama last week labeled a “network of death.”

In 2012, French intelligence tracked about 30 French citizens fighting in Syria. Valls, then interior minister, estimated the total at 600 when we met in 2013. “Now,” he says, “there are around 1,000. We think there may be 3,000 British citizens, some Germans, Italians and others,” who have until now been able to travel freely to Middle East war zones. Before the U.N. Security Council, Obama estimated the total number of foreign fighters at 15,000.

The 52-year-old Valls, in office for six months, has become a key figure in the international effort to check the spread and brutality of the Islamic State, al-Qaeda, their affiliates and other organizations.

He recently steered through Parliament a law restricting travel by French terrorism suspects that was a precursor to the U.N. resolution adopted unanimously last week. The resolution demands that all nations apply travel controls similar to those of France.

Valls’s brisk greeting for a visitor to his spacious Left Bank office suggested that neither his energy nor his determination has been dented as he has grappled with France’s growing military involvement in the war on Islamic terrorism in Iraq, a simmering tax revolt at home and challenges from his Socialist Party’s far left.

He showed no surprise when I began by asking about the possibility of a European 9/11. The threat had just been broached in an article in Le Nouvel Observateur magazine, written by Jean-Pierre Filiu, a top Middle Eastern scholar whose views are taken seriously in the prime minister’s office and elsewhere in the French government. “Daech is organized differently than was al-Qaeda in Afghanistan, where there were only a handful of Europeans,” Valls said, referring to the Islamic State by its French acronym. “We take great efforts to talk to the families of those who have gone to Syria, or who could be tempted to go,” he said — leaving no doubt that increased surveillance of terror suspects has been high on his agenda while in office.

As interior minister — and thus top cop — in the Socialist government of President François Hollande elected in 2012, Valls saw his approval ratings jump above 60 percent, thanks in part to his blunt, direct style and his Tony Blair-like emphasis from the left on law and order. Becoming prime minister “has not changed my DNA,” he says. “I have no intention of losing my identity.” He underlined the point by criticizing other European countries for not matching France’s efforts to sustain defense spending and project power abroad.

But the increase in responsibilities has changed one thing: those stratospheric approval ratings, which have been slashed in half. He was in fact promoted by Holland to counter the president’s own slide to record lows in popularity (13 percent) after his affair with an actress was splashed in humiliating detail in the media.

Valls walks a tight rope stretched between two volcanos — terrorism and a crushing tax burden that is resented by France’s working class as well as by the wealthy. He has enacted modest tax cuts, which he emphasized are part of Hollande’s own relatively new, more liberal policies.

There’s the rub: The more Valls succeeds, the greater threat he becomes to Hollande’s renomination in 2017. France’s two top Socialists are bound together by barbed wire, twisting together under the sullen gaze of angry French electors. They must not lose sight of the important battle against terrorists in which France now plays a leadership role.