 Mercer received word that Mawhood was leading his troops back across the bridge and back to Princeton . Mercer , on orders from Washington , moved his column to the right in order to hit the British before they could confront Washington 's main army . Mercer moved towards Mawhood 's rear but when he realized he would not be able to cut off Mawhood in time , he decided to join Sullivan . When Mawhood learned that Mercer was in his rear and moving to join Sullivan , Mawhood detached part of the 55th Regiment to join the 40th Regiment in the town and then moved the rest of the 55th , the 17th , fifty cavalry and two artillery pieces to attack Mercer . 
