Washington's activities from late 1778 to 1780 were more diplomatic and organizational, as his army remained outside New York, watching Sir Henry Clinton's army that occupied the city. Washington strategized with the French on how best to cooperate in actions against the British, leading to ultimately unsuccessful attempts to dislodge the British from Newport, Rhode Island and Savannah, Georgia. His attention was also drawn to the frontier war, which prompted the 1779 Continental Army expedition of John Sullivan into upstate New York. When General Clinton sent the turncoat General Benedict Arnold to raid in Virginia, Washington began to detach elements of his army to face the growing threat there. The arrival of Lord Cornwallis in Virginia after campaigning in the south presented Washington with an opportunity to strike a decisive blow. Washington's army and the French army moved south to face Cornwallis, and a cooperative French navy under Admiral de Grasse successfully disrupted British attempts to control of the Chesapeake Bay, completing the entrapment of Cornwallis, who surrendered after the Siege of Yorktown in October 1781. Although Yorktown marked the end of significant hostilities in North America, the British still occupied New York and other cities, so Washington had to maintain the army in the face of a bankrupt Congress and troops that were at times mutinous over conditions and pay. The army was formally disbanded after peace in 1783, and Washington resigned his commission as commander-in-chief on December 23, 1783.