Cyrus, the great Persian king, had conquered Babylon and other countries, as well, and he had kept on conquering until Persia ruled most of the world, all except Greece and Italy. About the Year 500 B.C. the new ruler of this vast Persian Empire was a man named Darius. Darius looked at the map, as you might do, and saw that he owned and ruled over a large part of it. What a pity, thought he, that there should be a little country like Greece that did not belong to him! So Darius said to himself, “I must have this piece of land called Greece to complete my empire.” Besides, the Greeks had given him some trouble. They had helped some of his subjects to rebel against him. Darius said, “I must punish these Greeks for what they have done and then just add their country to mine.” So he called his son-in-law and told him to go over to Greece and conquer it. His son-in-law did as he was told and started out with a fleet and an army to do the punishing. But before his fleet could reach Greece it was destroyed by a storm, and he had to go back home without having done anything. Darius was very angry at this, mad with his son-in-law and mad with the gods who he thought had wrecked his ships, and he made up his mind that he himself would go and do the punishing and conquering the next time. First, however, he sent his messengers to all the Greek cities and ordered each of them to send him some earth and some water as a sign that they would give him their land and become his subjects peaceably without a fight. Many Greek cities were so frightened by the threat of Darius and by his mighty power that they gave in at once and sent earth and water as they were told to do. But little Athens and little Sparta both hotly refused to do so, in spite of the fact that they were only two small cities against the vast empire of Darius. Athens took Darius’ messenger and threw him into a well, saying, “There is earth and water for you; help yourself”; and Sparta did likewise. Then these two cities joined their forces and called on all their neighbors to join with them to fight for their native land against Darius and Persia. So Darius made ready to conquer Athens and then Sparta. [Illustration: A Trireme.] In order to reach Athens his army had to be carried across the sea in boats. Of course, in those days there were no steamboats. Steamboats were invented thousands of years later. The only way to make a boat go was with sails or with oars. To make a large boat move with oars, it was necessary to have a great many rowers--three rows one above the other on each side of the boat.