Why thin armor could fit French cruiser doctrine

Why accept such thin belt armor on French treaty cruisers if it left them vulnerable in a gunfight?

Because they were designed for scouting, fast commerce attacks, withdrawal, and pack operations under treaty limits, not for independent close-range duels.

Ships such as the Suffren class were intended to scout, raid convoys, fire rapidly at range, and withdraw rather than trade blows in an isolated heavy-cruiser duel. Treaty displacement forced choices among armor, speed, endurance, and weapons, while Italy gained more latitude by exceeding declared limits. French cruisers were also expected to operate in packs with capital ships or allied battlecruisers. Judging one alone against every possible opponent therefore misreads the doctrine that shaped it.