The battleship has the most powerful artillery aboard. It is capable to destroy several smaller ships.

The destroyer has quick-firing guns only, but it allows to protect battleships against air attacks. Also, destroyers have sonars which make them ideal for hunting submarines.

Destroyers are anti-battleship units. If a quick destroyer moves close enough (2 hexagons) to a battleship it will attack with torpedos.

But a battleship has something to retaliate. The "Battleship secondary armament" skill allows a battleship to fire at destroyers automatically when they come close enough for a torpedo attack.

Best regards,
Panzer Strategy Team