Complex sentence: Pursuing the retreating Malcolm to Abernethy, William forced him to sign a peace treaty by which Malcolm became a vassal to his rival: a response to the harsh reality that the armed forces of Malcolm had met their match.
Split into simpler sentences: Pursuing the retreating Malcolm to Abernethy and forced him to sign a peace treaty where Malcolm to become a vassal to his rival. A response to the harsh reality that the armed forces of Malcolm had met their match.