In 1941-44, Captain McCann led two submarine squadrons, served in important positions at the Navy Department and was commanding officer of the battleship Iowa (BB-61) from August – November 1944. After promotion to the rank of Rear Admiral, he was Chief of Staff to Commander, Tenth Fleet, served with the Atlantic Fleet and Commanded Task Force 68, which escorted President Harry S. Truman to and from the Potsdam Conference.