Comment by cm2187
What the doctrine is and whether France would follow it in practice are two different things. But it shouldn't matter.
Nuclear weapons as an ultimate warning is the doctrine: https://en.wikipedia.org/wiki/Air-sol_moyenne_port%C3%A9e
Whether it would be followed in practice is anyone's guess. But place yourself in the shoes of the invading country. You are basically facing the same question. Would France ultimately use nuclear weapons? You may think that they will likely not. But if they do, that's a path to having your cities and basically your civilisation wiped out. So at what percentage probability are you still happy to try your luck at invading France? Unless you are a Hitler or Stalin, who were happy to spend tens of millions of their own population without a second thought, there is no scenario where a rational leader will be taking that risk. And therefore deterence is achieved.
I appreciate the nuanced response. France needs this capability to be publicly known for it to have the desired deterrent effect, and like most military expenditures, the intent to use it is somewhat independent from the capability to use it. I think France needed a new Maginot Line, and this is likely what that looks like in current year.
https://en.wikipedia.org/wiki/Maginot_Line