I was just told that because individuals in a nation have diverse interests, the "national interest" does not exist: I was trying to "homogenize" these different groups and treat them as if all of their interests were identical.
As pointed out many times here before, methodological individualism is nonsense, and pernicious nonsense to boot: it leads smart people to say silly things like the above.
Not every cell in my body needs to benefit from a course of action for that course of action to be in my self interest. My foot may need to be amputated for me to survive, but this is very bad for the cells in my foot. (They might have lived another month if left attached, but now they die immediately after the operation.) The interest of cancer cells in my body may run directly against the survival of my body.
Not every player on a sports team needs to benefit from a course of action for that action to be in the team's interest. Benching Callahan might be the best thing for the team, even if it signals that Callahan's career is over.
The people in a corporation may be there for a wide variety of reasons. Some of them may be corporate saboteurs. And many of them may be there just to get a paycheck, and not care at all about the survival of the corporation beyond the last day they are going to work there. Nevertheless, we can still reasonably talk about what is in the interest of the corporation: whatever helps the corporation to survive and become stronger is in its interest.
It is no different with the nation: that which promotes the survival of France as an entity is in France's interest, and that which retards that survival runs against its interest. It makes no difference at all that there may be many residents of France who are indifferent to its survival, or even wish it to disappear. (No difference at all in determining what is in France's interest: of course, if a lot of people inside France want France to disappear, it will have a harder time surviving!)