On 4 September not long after the birth of his and Peggy 's second son , Arnold 's force of over 1,700 men raided and burned New London and captured Fort Griswold , causing damage estimated at $ 500,000 . On September 4 , not long after the birth of his and Peggy 's second son , Arnold 's force of over 1,700 men raided and burned New London and captured Fort Griswold , causing damage estimated at $ 500,000 .