Complex sentence: At the outbreak of World War I, de Langle de Cary was given command of the Fourth Army, together with Pierre Ruffey and his Third Army, he was ordered to attack the Ardennes.
Split into simpler sentences: At the outbreak of World War I, de Langle de Cary was given command of the Fourth Army. In concert with Pierre Ruffey and his Third Army, he was ordered to attack the Ardennes.