Originally, Naval war was fought by ramming other ships and then jumping onto them to fight the crew. In Ancient Greece and Rome, the ships were usually powered by oars. During the Middle Ages, the cannon was invented. These could be shot at other ships from a great distance. Warships around this time also began to use sails instead of oars for power. In the middle 19th century, ships began to be plated in armor for protection. Steam engines allowed them move while carrying the armor, and better ones soon made them much faster. These early warships, called ironclads, are thought to have begun the type of Naval warfare used today. Ships soon became much larger.