During the Periclean period the Athenian democracy was supreme. Every matter that concerned the empire was discussed and decided by the popular assembly. Never before had any people enjoyed such perfect political liberty as did the citizens of Athens at this time, and never before were any people, through so intimate a knowledge of public affairs, so well able to direct the policies of state. Every citizen, it is affirmed, was qualified to hold civil office. PERICLES FOSTERS THE NAVAL POWER OF ATHENS.--Cimon's policy had been to keep the Grecian cities united in order that they might offer effectual resistance to the Persian power. The aim of his rival Pericles was to maintain Athens as the leading state in Hellas, and to oppose the pretensions of Sparta. Accordingly he encouraged the Athenians to strengthen their naval armament and to perfect themselves in naval discipline, for with Themistocles he was convinced that the supremacy of Athens must depend chiefly upon her fleet. As a part of his maritime policy, Pericles persuaded the Athenians to build what were known as the Long Walls,--great ramparts between four and five miles in length,--which united Athens to the ports of Piræus and Phalerum. Later, as a double security, a third wall was built parallel to the one running to the former harbor. By means of these walls Athens and her ports, with the intervening land, were converted into a vast fortified district, capable in time of war of holding the entire population of Attica. With her communication with the sea thus secured, and with a powerful navy at her command, Athens could bid defiance to her foes on sea and land. [Illustration: ATHENS AND THE LONG WALLS.] EVENTS LEADING UP TO THE THIRTY YEARS' TRUCE.--At the same time that Pericles was making the maritime supremacy of Athens more secure, he was endeavoring to build up for her a land empire in Central Greece. As her influence in this quarter increased, Sparta became more and more jealous, and strove to counteract it, chiefly by enhancing the power of Thebes. The contest between the two rivals was long and bitter. It was ended by the well-known Peace of Pericles, or the Thirty Years' Truce (445 B.C.). By the terms of this treaty each of the rival cities was left at the head of the confederation it had formed, but neither was to interfere with the subjects or allies of the other, while those cities of Hellas which were not yet members of either league were to be left free to join either according to choice. The real meaning of the Truce was that Athens gave up her ambition to establish a land empire, and was henceforth to be content with supremacy on the seas. It meant further that Greece was to remain a house divided against itself; that democratic Athens must share with aristocratic Sparta the hegemony, or leadership, of the Hellenic cities.