Lord Charles Cornwallis is most important in American history for the fact that he commanded the British army that was forced to surrender at Yorktown.  This surrender essentially ended the Revolution, allowing the American colonies to become free.  This is somewhat ironic because Cornwallis had generally taken positions that were favorable to the cause of the American colonists.   As a member of the House of Lords he had done things like voting against the Stamp Act and the Declaratory Act.  Even so, he served as an officer in the Revolutionary War.  It was during the war that he became a part of American history.