“We are striking Daesh in Syria because this terrorist organization prepares and organizes attacks in France from Syria, from these sanctuaries,” Mr. Valls said. “We are therefore acting in self-defense, which Article 51 of the United Nations Charter permits us to do.”

Mr. Hollande’s announcement did not come as a surprise. This month, Mr. Hollande said that France would start air surveillance missions over Syria, and a week later, his defense minister confirmed that France was gathering intelligence to conduct airstrikes on Islamic State targets there.

While France was already involved in an air campaign against the Islamic State in Iraq, it was initially reluctant to target the group in Syria out of fear of indirectly helping the Syrian president, Bashar al-Assad.

But the French government has shifted its position in the face of fears that the Islamic State is recruiting disaffected youths in France and sending or coordinating with European fighters back home to stage attacks.