Surface raiders complicate British convoy defense

Long-range aircraft added an air-defense requirement that a submarine-only convoy system did not need. Without guns or friendly fighters, a convoy could be shadowed while additional aircraft gathered for attack. A surface raider powerful enough to defeat a light cruiser forced Britain to assign at least a heavy cruiser for an even fight and preferably a capital ship for a favorable one. Each added threat changed the ships, weapons, and trained personnel that Britain had to distribute across the convoy system. Scharnhorst's combination of speed and sufficient gun power fit that threat-creation role without the cost of a much larger battleship. Its payoff lay not only in ships sunk but also in the disproportionate defensive force held away from other missions.