Navies have traditionally been organized by states for warfare at sea. Such warfare is essentially a contest about the sea lines of communication. During wars, these lines are used for transport of essential resources, power projection against foreign coasts and territories, or as an area for active defence against invasions. However, in the nineteenth and twentieth centuries, warfare became difficult to divide into warfare on land and at sea. It is not only the introduction of air power which made this division less useful.Grand strategy has always been about what to do with both land and sea forces during wars. Operations may often involve two or three armed services and the rapid development of new technology has meant that these services often also have to co-operate on the tactical level, that is, in combat with the enemy.