Don't be fooled. France is not aligning itself with America over Iran by it warning of war over Iran's nuclear ambitions.
France is just pointing out that its National Interest is now, by pure coincidence, in alignment with America's, much as Cancer can be in alignment with Uranus - and when that happens you need to be just as concerned.
France jealously guards its force de frappe and this is just part of that move. Iran having nuclear weapons dilutes that significantly. Please do not think that this is some selfless act or a realisation of realpolitik.
No, France is doing what it did during the Iraq war - protecting its National Interest. For France, realpolitik IS defending their National Interest. They have not just realised it.
In this aspect, France is very sensible. It is an utter disgrace that the UK and its F&CO are chock full of self-loathers on one hand or naive do-gooding imbeciles on the other.
Vive La France!