Complex sentence: The Battle of Messines was conducted by the British Second Army (General Herbert Plumer) and forced the German army to move reserves to Flanders from the Arras and Aisne fronts, which relieved pressure on the French.
Split into simpler sentences: The Battle of Messines was conducted by the British Second Army under the command of General Herbert Plumer. The offensive forced the German army to move reserves to Flanders from the Arras and Aisne fronts, which relieved pressure on the French.