Well, that didn't take long. Earlier today two or three muslims entered a Catholic church in Normandy and butchered an 84-year-old priest in a Halal-compliant manner (basically, throat cut while chanting Allahu akbar.)
The French president visited the area and said quite explicitly - France is at war. So that's official then.
How they will pursue the war is not obvious. They do have their aircraft carrier, the Charles de Gaule in the Gulf attacking ISIS strongholds in Syria and Iraq, but how do they conduct the war on French soil?
During WW2 'aliens' were interned, for example the USA rounded up all Japanese people in the States, including those with US nationality, and kept them in camps for the duration of the war. Individually they may have done nothing wrong, but collectively they could not be trusted. And to be fair, America experienced almost no attacks from Japanese in the continental United States. The policy worked.
Does France have the will to intern its muslim population? They already live in distinct areas called banlieux, roughly "suburbs". At the moment inhabitants are given free rein to travel in the rest of France - that would have to change. Most likely President Hollande does not have what it takes to make that happen. But there are elections next year and a certain Madame Le Pen would have - and the more these attacks keep happening the closer she gets to the Elysee Palace.