An Israeli flag hangs from a gate outside the Hyper Cacher in Saint-Mandé on Thursday, nearly a month after a gunman launched a siege inside the kosher market that left four Jewish hostages dead. (Leo Novel/for The Washington Post)

For all her 30 years, Jennifer Sebag has lived in a community that embodies everything modern Europe is supposed to be.

Inclusive, integrated, peaceful and prosperous, the elegant city of Saint-Mandé — hard against Paris’s eastern fringe — has been a haven for Jews like Sebag whose parents and grandparents were driven from their native North Africa decades ago by anti-Semitism.

“I’ve always told everyone that here, we are very protected. It’s like a small village,” Sebag said.

But in an instant on the afternoon of Jan. 9, Sebag’s refuge became a target. A gunman who would later say he was acting on behalf of the Islamic State walked into her neighborhood’s kosher market and opened fire, launching a siege that would leave four hostages dead — all of them Jewish.

A month later, the Jews of Saint-Mandé are planning for a possible exodus from what had once appeared to be the promised land.

In homes, in shops and in synagogues guarded night and day by soldiers wielding assault rifles, conversations are dominated by an agonizing choice: stay in France and risk becoming the victim of the next attack by Islamic extremists, or leave behind a country and a community that Jews say they are proud to call home.

The French government has scrambled to persuade them not to go, aware that if Jews see little future for themselves in Saint-Mandé — where Muslims, Christians and Jews have long lived in harmony — then there’s no chance for the European ideal of interfaith coexistence.

And yet, for a rapidly rising number of Jews, here in Saint-Mandé and across France, the decision has already become clear.

“The question is not will they leave or won’t they leave,” said Alain Assouline, a prominent Saint-Mandé doctor and president of a Jewish community center. “The question has become when they will leave.”

For Sebag, her husband and their three young sons, the answer is within months. After pondering a move for economic reasons, the attack on a market where they regularly shop erased all doubts.

They will travel this summer from the only home they have ever known to Israel, where they have no close friends or relatives, where they don’t speak the language, and where war flares all too regularly. There they will start anew, much as Sebag’s grandparents did decades ago.

“They came here from Morocco and Tunisia because France was a wonderful country,” said Sebag, a cheery real estate agent who lives with her family in an airy, pre­war apartment overlooking one of Saint-Mandé’s chic shopping districts. “They made all sorts of sacrifices, and we’ve had a really nice life here — until today.”

Rabbi Marc Krief of the Synagogue of Vincennes–Saint-Mandé says he doesn’t want his congregants to flee France in fear when the scourge of anti-Semitism is global. (Leo Novel/for The Washington Post)

The attack on the kosher market was the last in a three-day series of radical Islamist assaults that traumatized the nation. By the end, 17 victims lay dead, including much of the staff at the satirical newspaper Charlie Hebdo.

But of all the communities affected, France’s half-million Jews have perhaps felt the consequences most acutely.

French Jews were already on edge by the time Amedy Coulibaly, a 32-year-old small-time criminal and son of Malian immigrants, took hostages at the Hyper Cacher grocery on the border of Paris and Saint-Mandé.

Anti-Semitism had been rising in France, as it had across Europe. In Britain last year, for instance, there were more than 1,100 anti-Semitic incidents recorded, double the number from 2013, according to data released Thursday by the Jewish nonprofit Community Security Trust.

But the fears of rising violence have been especially pronounced in France after a 2012 attack at a Jewish school in Toulouse that left a teacher and three students dead.

The Jewish Agency, which encourages immigration to Israel, says the number of French Jews leaving for Israel each year had been steady at about 2,000 until 2013, when it hit 3,400. Last year, it jumped to more than 7,000 — making France the leading contributor of immigrants to Israel and marking the first time that more than 1 percent of a Western nation’s Jewish population has left for Israel in a single year, according to Avi Mayer, a spokesman for the Jewish Agency.

Since the Hyper Cacher attack, calls to the Jewish Agency’s Paris office have more than tripled, Mayer said, and the agency is predicting that 15,000 French Jews will move to Israel in 2015.

Many others will choose to leave for the United States, Britain, Canada, Australia and beyond.

At the kosher butcher’s shop two doors down from the still-shuttered Hyper Cacher one recent day, the talk focused on whether to go, and where.

“My husband’s ready, but not me,” a young woman picking up a chicken told the butcher. “I was in Tel Aviv in July, and I watched rockets fly into the sea. I wouldn’t feel safe there, either.”

The butcher, a 20-year-old named Aaron Sultan, said he and his fiancee are deciding where to start their life together and are leaning toward Israel.

“My parents left Tunisia during the Yom Kippur War [in 1973]. My mom tells the story that they fled for France when the Arabs were at their door, ready to kill them,” said Sultan, who wears a black kippah, or prayer cap, atop his close-cropped dark-brown hair.

Now he is preparing to flee France, but his parents are reluctant. “I’ve asked my mom, ‘Do we wait for the same thing here? Until the Arabs are at our door, ready to kill us?’ ” said Sultan, who spent the afternoon of the attack hiding on the shop floor as the crack of bullets pierced the air a few yards away. “It’s hard to leave, but when we don’t feel safe, we have no choice.”

The government has tried to reassure the country’s Jews by dispatching more than 10,000 camouflage-clad troops to guard “sensitive sites,” including synagogues and Jewish schools and community centers. Three soldiers guarding one such center were attacked Tuesday by a knife-wielding assailant in the southern city of Nice.

Far from comforting, the troops’ presence has become for many Jews a symbol of their vulnerability.

“It’s more stressful than reassuring,” said Sebag, who walks past the troops each day as she drops her kids at preschool. Even with all the threats facing Israel, she notes, soldiers are not regularly deployed to defend toddlers.

And yet, Saint-Mandé Mayor Patrick Beaudoin said, the country also needs to defend its Jewish population at all costs. “They belong to this country. France needs them,” he said.

A mass exodus from Saint-Mandé could be ruinous for a city where about a third of its 22,000 residents are Jewish and where the faith’s roots run deep. The formidable white stone walls of one of the area’s main synagogues have been standing for the past century.

The community has changed in recent years, with the original Ashkenazi Jews — those with European origins — supplemented by an influx of Sephardic Jews from North Africa.

Muslims from North Africa have also begun to make the area home, adding to a national Muslim population of about 5 million, though their community in Saint-Mandé is considerably smaller than the Jewish one. By nearly all accounts, the new arrivals have been welcomed to the city, with Jews and Muslims befriending one another and going into business together. Assouline, the doctor and Jewish community center leader, has two partners in his practice: one Catholic, the other Muslim.

Jewish residents of Saint-Mandé say the problem of Islamic extremism doesn’t exist here. But as they discovered Jan. 9, it’s not far away, either, lurking in the less-salubrious suburbs, where last month’s attackers had their roots.

“We can’t say that these are jihadists imported from Iraq or Syria,” said Marc Krief, rabbi at the Synagogue of Vincennes - Saint-Mandé. “They were French citizens. They grew up in the suburbs. They went to the local mosques. They learned their way of thinking from here.”

Krief said he has told his congregants that if they want to leave France for economic or cultural reasons, they should go ahead. But he does not want them fleeing in fear when the scourge of anti-Semitism is global.

“I don’t see a country in the world that’s safe enough,” Krief said. “In Israel, there’s war. In the United States, there could be another terrorist attack. It wouldn’t change anything to leave.”

And yet, given the lessons of Jewish history, the impulse to leave Europe amid increasingly ominous warning signs runs strong.

“Personally, I have faith in our community. I’m an optimist,” said Assouline, who intends to stay. “But whenever I say that, there’s always someone who reminds me, ‘In 1933, there were two types of Jews: the pessimists and the optimists. The pessimists left and went to the U.S. The optimists ended up in the death camps.’ ”

Cléophée Demoustier contributed to this report.