What was significant about the second battle of Ypres

Second Battle of Ypres, (April 22–May 25, 1915), second of three costly battles in World War I at Ypres (now Ieper), in western Flanders. The battle marked the Germans' first use of poison gas as a weapon. Although the gas attack opened a wide hole in the Allied line, the Germans failed to exploit that advantage.

That's true, it's a belief in one god