Means by which a nation extends its military power onto the seas. Measured in terms of a nation's capacity to use the seas in defiance of rivals, it consists of combat vessels and weapons, auxiliary craft, commercial shipping, bases, and trained personnel. It includes aircraft based on carriers or used in support of shipping. Its main purpose is to protect friendly shipping from enemy attack and to destroy or hinder the enemy's shipping. It may also be used to enforce a blockade. Finally, naval forces have been used to bombard land targets from the sea. The aircraft carrier added a new dimension to this capability, as did the missile-firing nuclear submarine. The classic exposition of the role of sea power as the basis of national greatness is Alfred Thayer Mahan's The Influence of Sea Power upon History (1890).