From today’s perspective, it is difficult to appreciate the long odds faced by Americans and their French allies in the Yorktown campaign of 1781. The prospects for George Washington and his northern army at the start of the year were not bright. They were keeping a lonely vigil outside of New York City, monitoring the actions of Sir Henry Clinton's vastly superior British forces.
The situation in the South also was dismal. The Americans had won an important victory at Cowpens (January 1781), but later suffered a string of defeats. The British commander, Lord Charles Cornwallis, completed his sweep through the Carolinas and in May entered Virginia to root out the sources of American resistance.
Further, the American populace was weary of the war, the government was broke and soldiers in all theaters were not inclined to commit to lengthy terms of service.
Nonetheless, it became evident to Washington and his French allies that a possible opportunity was developing to trap Cornwallis’s army in a position where they could not be resupplied or reinforced. To accomplish this, however, would require moving two armies, one American and the other French, over 450 miles to Virginia.
If those movements were detected early on by the British, then the scheme would collapse and a possibly disastrous encounter would occur in the North. Further risk was provided by the uncertainty of French naval aid. If Chesapeake Bay could not be temporarily sealed off from the British fleet, then Cornwallis would be reinforced by sea and stand an excellent chance of defeating his exhausted opponent in Virginia.
The gamble paid off for Washington. Events in different locations evolved in a manner that enabled the allied armies to march safely to their destination, while the French navy managed to secure the Chesapeake for a brief time before a British relief fleet arrived. These events included: