The British had a plan to win the war after the Declaration of Independence was sent to King George III in 1776. They wanted to cut New England off from the rest of the colonies. They planned to do this by seizing New York.
George Washington and the newly formed Continental Army moved to New York to prepare to defend the city. In August 1776, the British Royal Army landed at Kip's Bay in New York and defeated the American Patriots. Washington and the Continental Army were forced to retreat and the British began their occupation of New York City.
After the British took New York, George Washington and his men retreated through New Jersey back to Philadelphia where the Continental Congress was stationed. The British, controlling New York, easily took control of New Jersey as well.
Morale for the Continental Army was low and Washington knew he needed to do something. He regrouped his men and decided he needed to defend Philadelphia by returning to New Jersey and going on the attack. George Washington crossed the Delaware River on Christmas Day, 1776 and began marching into New Jersey. Washington and his men launched surprise attacks in the New Jersey towns of Princeton and Trenton.
The American Patriots, led by George Washington, won the battles. The Battles of Trenton and Princeton were a boost to the morale of the Patriot cause, leading many recruits to join the Continental Army in the Spring of 1777.