PARIS — The three young Frenchmen were arrested as they tried to make their way to Syria to wage jihad. They had not harmed anyone in France or made plans to do so, according to the evidence at their trial in January, but in France these days, seeking to fight in Syria is enough to bring a charge of plotting terrorism — and in this case sentences of three to five years in prison.

France, and much of Europe, have grown steadily more concerned over the past year about the possibility that the main terrorist threat could come from their own citizens, European passport holders who can move relatively easily between their homelands and the battlefields of Syria, where Islamist rebel groups are fighting the government of President Bashar al-Assad.

In that climate, France is becoming especially aggressive by arresting would-be jihadis even before they leave the country or set foot on a battlefield.

France’s fears came to the fore on Sunday when officials announced the apprehension of a suspect in the killings of three people at the Jewish Museum in Brussels last month, a 29-year-old Frenchman said to have spent time in Syria last year.