The schooner remained in that vicinity, ready to help to defend the seat of the Federal Government in the event that Robert E. Lee's army — which had crossed the Potomac River into Maryland — attack. After the Union stand at Antietam had repelled this invasion of the North, "Arletta" left Washington on 18 September to begin operations downstream with the Potomac Flotilla. She continued this duty until returning to the Washington Navy Yard at the end of October to have her mortar removed and to be fitted out as an ordnance vessel.