Complex sentence: He served as aide de camp to Sir Douglas Haig, commander of the British Expeditionary Force from 1917 to 191, and looked after the Minister of Munitions on several trips to the front during the war.
Split into simpler sentences: He served as aide de camp to Sir Douglas Haig, commander of the British Expeditionary Force from 1917 to 1918. He looked after the Minister of Munitions on several trips to the front during the war.