<P> De Grasse received these letters in July at roughly the same time Cornwallis was preparing to occupy Yorktown, Virginia . De Grasse concurred with Rochambeau and subsequently sent a dispatch indicating that he would reach the Chesapeake at the end of August but that agreements with the Spanish meant he could only stay until mid-October . The arrival of his dispatches prompted the Franco - American army to begin a march for Virginia . De Grasse reached the Chesapeake as planned, and his troops were sent to assist Lafayette's army in the blockade of Cornwallis . A British fleet sent to confront de Grasse's control of the Chesapeake was defeated by the French on September 5 at the Battle of the Chesapeake, and the Newport fleet delivered the French siege train to complete the allied military arrival . The Siege of Yorktown and following surrender by Cornwallis on October 19 were decisive in ending major hostilities in North America . </P> <P> In October 1781, a plan had been worked out between de Grasse, commander of the French fleet in the West Indies, and Francisco Saavedra de Sangronis, General Bureau for the Spanish Indies, court representative and aide to the Spanish Governor of Louisiana, Bernardo de Gálvez . The strategic objectives of this plan were to guide the Franco - Spanish military forces in the West Indies to accomplish the following objectives: </P> <Ul> <Li> Aid the Americans and defeat the British naval squadron at New York, </Li> <Li> Capture the British Windward Islands, and </Li> <Li> Conquer the island of Jamaica . </Li> </Ul> <Li> Aid the Americans and defeat the British naval squadron at New York, </Li>

When did great britian and france declare war