collaborer

Collaborer usually just means to collaborate or work together on something (collaborer sur un projet), but in a French historical context it carries a second, heavily loaded meaning: to collaborate with an occupying enemy, specifically the Vichy regime's cooperation with Nazi Germany during WWII.

• Les deux entreprises collaborent sur un nouveau projet. — The two companies are collaborating on a new project.
• Elle a collaboré avec plusieurs chercheurs internationaux. — She has collaborated with several international researchers.
• Pendant l'Occupation, certains ont choisi de collaborer avec l'ennemi. — During the Occupation, some chose to collaborate with the enemy.

In a historical or political sentence, collaborer (and especially the noun collaborateur) carries a strong negative charge in French tied specifically to WWII — using it loosely about ordinary teamwork in that era-adjacent context can sound like an accusation, not a compliment.

• politics La Collaboration — Verified: the term for the Vichy regime's policy of cooperating with Nazi Germany during the 1940–1944 Occupation, sealed publicly when Pétain declared "j'entre aujourd'hui dans la voie de la collaboration" after meeting Hitler at Montoire on October 24, 1940.