Why did the British fight the Americans in the Battle of Princeton in 1777?

The American Colonists were fighting a revolution against the British. They fought the Battle of Trenton, NJ on Christmas Day, because General George Washington needed to win a big battle, he had been beaten quite a few times by the British recently, he needed to get soldiers whose enlistments were about to expire to reenlist, he needed to prove to the Colonial government in Philadelphia he could win a revolution. The British did not expect a battle on Christmas Day, or in the middle of winter. They thought they had all but beaten George Washington and the Colonial Army. Gen. Washington crossed the icy Delaware river from Pennsylvania to New Jersey and caught the British off guard at Trenton. The victory had the consequences General/President George Washington wanted. It was one the major turning points of the Revolution, along with the Battle of Saratoga and Battle of Yorktown.