During the last half-century the art of naval warfare had made great progress in Greece. The Greek war-galley, or trireme, a vessel propelled by three banks of oars, had always been furnished with a sharp-pointed prow, for the purpose of ramming an opponent's ship; but many years elapsed before the Greeks attained genuine skill in the use of this formidable weapon. According to the ordinary method of fighting, after the first shock of collision the affair was decided by the hoplites, or heavy-armed infantry, stationed on the decks of the two contending ships; and in this manner was fought the engagement between the Corcyraean and Corinthian fleets which occurred in the year before the outbreak of the Peloponnesian War. There the ship was simply a vehicle, which served to bring the antagonists together, and the rest was left to the prowess of the hoplites. The Athenians were the first to abandon this crude and clumsy style of fighting, and in the course of two generations their seamen had become renowned throughout Greece for the unrivalled skill which they showed in working and manoeuvring the trireme. A few hoplites were still carried, to serve in cases of emergency; but by far the most important part in the encounter was played by the trireme itself, with its long, tapering, sharp-pointed prow. To use this deadly but delicate instrument with effect required great coolness, dexterity, and judgment, on the part of the steersman, and a crew under perfect command. The tactics usually employed were as follows: watching his opportunity, the captain gave the order "full speed ahead!" and darting rapidly through the enemy's line, wheeled suddenly round, and drove the beak of his galley with terrible force against the stern or side of the vessel selected for attack. One blow from the long lance-like point, propelled by the whole weight and impetus of the trireme, was sufficient to sink or disable an enemy's ship, and the attacking galley was then backed away from the wreck, and directed against another victim. The incessant practice of nearly half a century had enabled the Athenians to attain consummate mastery in this new method of naval warfare; and they were now to give signal proof of their immense superiority over the other maritime powers of Greece. In the same summer which witnessed the investment of Plataea, the Spartans planned an expedition against Acarnania, the westernmost province of Greece, which they wished to detach from the Athenian alliance. A Spartan officer, named Cnemus, was sent off in advance, with a thousand hoplites, to raise the wild mountain tribes, and led an attack against Stratus, the capital of Acarnania; and in the meantime orders were sent round to equip a numerous fleet, which was to support the operations of Stratus by harassing the coast districts.