Complex sentence: American and French armies that had massed outside New York City began moving south late in August, and arrived near Yorktown in mid-September; deceptions about their movement successfully delayed attempts by Clinton to send more troops to Cornwallis.
Split into simpler sentences: American and French armies that had massed outside New York City began moving south in late August, and arrived near Yorktown in mid-September. Deceptions about their movement successfully delayed attempts by Clinton to send more troops to Cornwallis.