On January 2, 1777, Washington didn't know entirely what to do. He convened a council of war with his officers. Washington had been alerted that the road to Princeton was undefended. He had a plan of marching deep into New Jersey and the consensus from the council was to keep going.
Most of Washington's army marched northeast. Residents loyal to Washington relayed information that the town of Princeton was open to attack on the east side of the town. As January 3 began, the Continentals were about two miles from Princeton. Washington ordered Brigadier General Hugh Mercer to march to the left and destroy a bridge on a road that led to Trenton. The bulk of the force, led by Major General John Sullivan's division, marched on Princeton. Washington's total forces were about 4,500 men.
The British forces were led by Lieutenant Colonel Charles Mawhood. He had about 1,200 men. Mawhood's mission was to keep Princeton secure for General Cornwallis. Princeton was key for the British communications route to New York City.
Mawhood spotted Washington's front column and ordered his force back inside Princeton. Fighting ensued and Mawhood had Mercer's troops attacked by highly skilled British infantrymen. Mercer was overrun and mortally wounded. Washington pressed the engagement and entered the town. Sullivan did not risk a frontal attack and found a stalemate happen until troops, personally led by Washington, broke the British line.
Both the Battles of Trenton and Princeton split the British Army in two and gave the people, loyal to independence, new hope and confidence that the war could be won.
Sources for this post: Battle of Princeton on mountvernon.org
10 Facts About the Battle of Princeton on mountvernon.org