Complex sentence: The Battle of Messines took place on the Western front of the First World War on 7 June 1917, when the British Second Army under the command of General Herbert Plumer launched an offensive near the village of Mesen (Messines) in West Flanders, Belgium.
Split into simpler sentences: The Battle of Messines took place on the Western front of the First World War. It began on 7 June 1917 when the British Second Army under the command of General Herbert Plumer launched an offensive near the village of Mesen (Messines) in West Flanders, Belgium.