Complex sentence: In April 1951, the Belgians fought in one of the key battles of the Korean War--the Battle of the Imjin River.The First Battalion was relieved by a new battalion fresh from Belgium in August 1951 which stayed until 1955.
Split into simpler sentences: In April 1951, the Belgians fought in one of the key battles of the Korean War--the Battle of the Imjin River. In August, the First Battalion was relieved by a new battalion fresh from Belgium in August 1951 which stayed until 1955.