Complex sentence: Rose's education was interrupted by the First World War and as he was already enrolled in the Highland Territorial Division with his school friends, he was enlisted on the first day of the war and subsequently posted to to Gallipoli.
Split into simpler sentences: Rose's education was interrupted by the First World War. As he was already enrolled in the Highland Territorial Division with his school friends, he was enlisted on the first day of the war and subsequently posted to to Gallipoli.