People go through security checks before entering the Christmas market in Strasbourg, France, on Dec. 20. (Jean-Francois Badias/AP)

When a massive truck plowed through crowds at a Berlin Christmas market Monday night — killing at least 12 in what German authorities are now calling a likely terrorist attack — the incident immediately raised questions over security in a Europe where militant-driven bloodshed has become a fact of life.

As investigators explored how such a large vehicle could so easily have made its way into a pedestrian square, leaders across the continent weighed how best to balance the open spirit of public events such as Christmas markets with a need for heightened security. There were no easy answers.

The Christmas market is one of the hallmarks of European holiday cheer — a tradition that, for centuries, has brought revelers out into the cold to enjoy mulled wine, traditional sweets and craftsman goods. But with bustling crowds crammed into tight, narrow spaces, they are also natural targets for potential terrorist plots. Worse, they are difficult — and maybe even impossible — to protect, as they stretch across busy urban plazas and into side streets and alleys.

This sense of vulnerability is particularly acute in France, which has suffered the worst of the terrorist violence in Europe in the past two years. More than 230 have died here since January 2015 — including 86 in a similar truck attack in July, when a man inspired by the Islamic State drove through crowds gathered in Nice to celebrate Bastille Day, France’s national holiday.

46 years of terrorist attacks in Europe, visualized

Although Christmas markets originated in Germany, they are also popular in France, with several in Paris and others in virtually every major French city and medium-size town.

On Tuesday, President François Hollande quickly reassured French citizens that heightened safety measures had already been put in place for the holiday season.

“We have a high level of threat,” he said, “and we also have a particularly high level of mobilization and vigilance.”

Last month, French authorities said they foiled a terrorist plot that would have targeted, among other places, the market along Paris’s Champs-Élysées, the grandest boulevard in the French capital.

After several arrests were made in November in Strasbourg, home to France’s largest Christmas market, the eastern French city has been on high alert, even threatening to cancel its famous holiday event if it received significant threats.

Bruno le Roux, France’s interior minister, told Le Monde newspaper Tuesday that his office had already ordered an increase in police patrols and soldiers in the country’s famed Sentinelle squadron during the last 10 days of the year. Already, an increased police presence could be felt at the markets in central Paris, where officers patrolled as shoppers and tourists paused at stalls to browse for holiday gifts.

Unlike other public events — such as the giant watch parties for the Euro 2016 soccer tournament held this summer or the “Paris Plages,” beaches created on the banks of the Seine River — Christmas markets are not typically held in separate enclosures with entrances and exits.

Instead, they are often staged in large public squares or along central pedestrian thoroughfares, where commuters and passersby can linger as they go about their daily routines. There are usually no security checks to enter and exit the markets.

François Heisbourg, a former member of a French presidential commission on defense and national security, said there is little to be done aside from simple measures like concrete barriers that, in theory, could prevent large vehicles from entering crowded areas.

German authorities confirmed to The Washington Post that no such barriers had been placed outside the Berlin market.

But the public, pedestrian nature of these events posed what Heisbourg said was virtually an impossible challenge.

“If you want to kill people on Fifth Avenue at Christmastime and mow down people with an assault rifle, there is really nothing out there which is going to prevent you from doing that — not in Europe, and certainly not in the United States.”