The power of the sea
The Spartans have a certain kind of strength, the physical prowess of their soldiers. If you left an Athenian and a Spartan in a ring, chances are the Spartan would win. But that's what makes the Athenian navy such an important part of the book. The power of the sea symbolizes the strategy of coordinating with nature for attacks. This is something that comes up again in Sun Tzu's The Art of War where he discusses harnessing the natural advantages of the setting.
The futility of siege
Another critical strategy of warfare during this time was to lay siege to important cities. Perhaps the army is not strong enough to invade and conquer a city and to establish a base there, but if an army is strong enough just to surround the city and cut them off, to stop supplies from entering, then they can be drained over time of their resources. The Athenian navy makes this strategy futile by harnessing the ports. As long as people move to the ports, they can get what they need. This shows Pericles's strategic power.
The empty invasion
Pericles's decision to move the civilians to the ports also leads to this delightful image. The Spartans come in full regalia, ready to conquer, and they're pumped up like a football team before the kickoff—only to be completely confused and deflated when they realize all the towns are abandoned. This is a symbol to them that they are losing the war because their intelligence is so lacking that their physical strength is to no avail. As a symbol, the crisis means that damage can be avoided with critical thinking.
The peace treaties
The treaties symbolize defeat because the Spartans declared war originally and then asked for the war to end. However, to the Athenians, the treaty is only temporary because they know that, if the Spartans had more resources, they would continue the war. Then they attack Sparta a few years later, indicating that they suspect Sparta is up to something. This can be seen as a symbol for the damage a war can have throughout time, because even after the war is over, both sides are too paranoid to let the other nation thrive.
Disease as a symbol
If not careful, one can begin to suspect that a war boils down to thinks humans can control, like training, strategy, fearless execution, and skill. Those are inextricable parts of warfare, to be sure, but disease shows that some parts of war are unpredictable. When the plague takes Pericles away in death, the war changes drastically. To a Greek soldier at the time, this would be a threatening portent. As a symbol, disease shows that fate still has its place in war.
