During the period after the Persian war two great powers arose in Greece, which were destined to come into close and virulent conflict. These were the league of Delos, which developed into the empire of Athens, and the Peloponnesian confederacy, under the leadership of Sparta. The first of these was mainly an island empire, the second a mainland league; the first a group of democratic, the second one of aristocratic, states; the first a power with dominion over the seas, the second a power whose strength lay in its army. Such were the two rival confederacies into which Greece gradually divided, and between which hostile sentiment grew stronger year after year. It became apparent as the years went on that a struggle was coming for supremacy in Greece. Outbreaks of active hostility between the rival powers from time to time took place. At length the situation grew so strained that a general conflict began, that devastating Peloponnesian war which for nearly thirty years desolated Greece, and which ended in the ruin of Athens, the home of poetry and art, and the supremacy of Sparta, the native school of war. The first great conflict of the Hellenic people, the Persian war, had made Greece powerful and glorious. The second great conflict, the Peloponnesian war, brought Greece to the verge of ruin, and destroyed that Athenian supremacy in which lay the true path of progress for that fair land. In 431 B.C. the war broke out. Sparta and her allies declared war against Athens on the ground that that city was growing too great and grasping, and an army marched from the Peloponnesus northward to invade the Attic state. Meanwhile the Athenians, under the shrewd advice of Pericles, adopted a wise policy. It was with her fleet that Athens had defeated Persia, and her wise statesman advised that she should devote herself to the dominion of the sea, and leave to Sparta that of the land. Their walls would protect her people, their ships would bring them food from afar, they were not a fair match for Sparta on land, and could safely leave to that city of warriors the temporary dominion of Attic soil. This advice was taken. When the Spartan army came near Attica all its people left their fields and homes and sought refuge, as once before, within the walls of their capacious capital city. Over the Attic plain marched the invaders, destroying the summer crops, burning the farmers' homesteads, yet recoiling in helpless rage before those strong walls behind which lay the whole population of the state. From the city, as we know, long and high walls stretched away to the sea and invested the seaport town of Piraeus, within whose harbor lay the powerful Athenian fleet. And in the treasury of the city rested an abundant supply of money,--the sinews of war,--with whose aid food and supplies could be brought from over the seas. In vain, then, did Sparta ravage the fields of Attica. The people of that desolated realm defied them from behind their city walls.