Of all the car manufacturers to participate in World War One, Renault was by some margin the most active.
Like Rolls-Royce, Renault did initially contribute to the French war effort by supplying cars to the military. More impressively, every single Renault taxi cab in Paris was temporarily repurposed as a troop transport in 1914, to help counter the German offensive during the First Battle of the Marne.
As important as the commandeered cabbies were, of even more significance to the war was Renault’s FT light tank. Although no-where near as heavily armed or as imposing as the British heavy tanks, their (relative) speed and the sheer quantity of them made this dinky little device a devastatingly effective asset.