The Marne and Yser rivers were strategic during World War I. The Marne is a river in France that runs for about 500 miles, while the Yser is a river in Belgium that runs for about 60 miles. Both rivers played a significant role in the war, as they were used to defend against German advances. The Battle of the Marne was fought in 1914, and the Battle of the Yser was fought in 1915. Both battles resulted in Allied victories, and helped to turn the tide of the war.