Complex sentence: The Battle of Poelcappelle began on 9 October, was costly to both sides and most of the ground that the British captured opposite Passchendaele, was lost later in the day to German counter-attacks.
Split into simpler sentences: The Battle of Poelcappelle began on 9 October, causing many casualties to both sides. Most of the ground that the British captured opposite Passchendaele was lost later in the day to German counter-attacks.