WWI Battle Sites include Ypres, Verdun, Somme, Argonne, and Marne. Ypres was the site of three major battles during WWI, including the Second Battle of Ypres in 1915, which saw the first use of poison gas on the Western Front. Verdun was the longest battle of WWI, lasting from February to December 1916. The Battle of the Somme was one of the largest battles of WWI, with over 1 million casualties. The Battle of Argonne was the final Allied offensive of WWI, lasting from September to November 1918. The Battle of the Marne was a major turning point in WWI, resulting in a German retreat and the beginning of trench warfare.