Complex sentence: Don Pedro Vique was on board the "Santiago" when the attack began and he immediately drew in to the beach and landed at the head of a troop of cavalry, carried on board as a mobile striking force.
Split into simpler sentences: Don Pedro Vique was on board the "Santiago" when the attack began. He immediately drew in to the beach and landed at the head of a troop of cavalry, carried on board as a mobile striking force.