With victory, came spoils. Tons of French war material captured by the Nazis, uniforms, tanks, artillery...and helmets...Adrian helmets. The Casque Adrian, was designed by General August-Louis Adrian using design elements from helmets firefighters were wearing in Paris. Produced in 1915, this new helmet would hopefully allay the horrific conditions of war on the Western Front. Cloth hats don't stop metal...and although a direct hit would pierce the helmet and still kill the wearer, having a shell of steel about your head could ward off a bit of shrapnel or the stray bullet.
There are two versions of the French Adrian helmet, the Modele 1915 and the Modele 1926. The differences between the two are in the construction. The WWI helmet is constructed of five separate stamped steel parts riveted together, the body, front and rear visor, the emblem on the front, and the comb that went on top to reinforce the helmet's strength. The later design, the Mle. 1926 was reduced to three parts, a fully stamped steel shell and a comb stamped from aluminum and the front insignia.
Nazi Germany did not let these helmets go to waste. It appears that most of the French helmets captured by the Germans were the Model 1926, however, it is conceivable that Model 1915 helmets were also recovered, but there's not enough photographic evidence or extant examples to show the Mle. 1915 being utilized by the Nazis. There is one exception, and we'll get to that towards the end. Adrians were issued to the Germans to support troops only. No front-line German soldier was given a French helmet to wear. However, it took most of Germany's on-hand stock of their own just to outfit the heads of their fighting men, because they didn't have enough for the men in the rear. This was an issue for the Nazis throughout the entirety of WWII. Between 1935 and 1937, before Germany's entrance into the Second World War, Nationalist China orders 220,000 German M1935 helmets. The existing stocks were depleted, and the one major company producing the new helmet for the German armed forces, Eisenhüttenwerk AG, struggled to completely fill the order, with newly manufactured supplies going to China, and not the Wehrmacht. This is why many photographs of German soldiers early in WWII show front-line German troops wearing a German helmet from the last war, the Model 1916 Stahlhelm which was re-worked and re-issued to Hitler's Army.
With that said, you will see French helmets being worn by the German Luftshutz (essentially air-raid wardens), The German Red Cross, and the Volkssturm, Germany's last ditch army of old men, women, boys, and girls...