Well, I'll accept as a general matter that they're not stupid, but there are times when intelligence, and when prescience, fails you, and this was that time for France. Chirac over-reached -- he thought he could bluster and scuttle the US plans, and come out looking like a leader. He gambled that Bush was weak, but Chirac was wrong.
The net of this is that Chirac thought he could stand up to the US and that the US would stand down. He would earn the gratitude of Saddam, and the economic benefits from that gratitude, and also boost the status of the European Union in its gambit to sit equal to the US on the world stage.
And Chirac was wrong, and he caused a war rather than prevented one.