In the early years of the war Washington was often in the middle of the action, first directing the Siege of Boston to its successful conclusion, but then losing New York City and almost losing New Jersey before winning surprising and decisive victories at Trenton and Princeton at the end of the 1776 campaign season. At the end of the year in both 1775 and 1776, he had to deal with expiring enlistments, since the Congress had only authorized the army's existence for single years. With the 1777 establishment of a more permanent army structure and the introduction of three-year enlistments, Washington built a reliable stable of experienced troops, although hard currency and supplies of all types were difficult to come by. In 1777 Washington was again defeated in the defense of Philadelphia, but sent critical support to Horatio Gates that made the defeat of Burgoyne at Saratoga possible. Following a difficult winter at Valley Forge and the entry of France into the war in 1778, Washington followed the British army as it withdrew from Philadelphia back to New York, and fought an ultimately inconclusive battle at Monmouth Court House in New Jersey.