At the start, Athens seemed highly likely to win the war. They held nearly every advantage and could not be seriously harmed without radical shocks to the geopolitical system. Indeed, even after the plague and the death of Perikles, the peace that eventually ended the first phase of the war (431-421 BC) represents a modest Athenian victory. It's not until the final phase (414-404 BC) that Sparta gained the initiative and Athenian power collapsed. Even then, it took massive Persian funding and a nine-year war of attrition to finally force Athens' unconditional surrender.

The historian Thucydides - our main source for the first 2/3rds of the conflict - explains exactly what gave Athens the edge at the start: a powerful navy and a huge cash reserve. As long as it could maintain naval supremacy, Athens would be able to keep its subject allies in line, deploy its militia wherever it wanted, and import food from abroad to keep its people fed. This meant that not only was Athens functionally invulnerable (Sparta would never be able to starve it into submission unless it could challenge Athens at sea), but it also had complete freedom to choose its targets and fight the war on its terms.

Sparta, by contrast, was weak. It had no public treasury and no fleet to speak of. It usually required its subject allies only to provide men for its armies (not money). These men were badly needed, because the Spartan militia was getting dangerously small at this time due to the decline of Spartan citizen numbers; even in the Peloponnese there were several other states that could easily match Spartan numbers. Sparta relied on its allies to provide the manpower that gave them their only real advantage - a larger allied land army than that of Athens. But continued Spartan inaction despite repeated pleas from the allies for help against Athenian expansion had made its alliance unstable. There was a real question whether Peloponnesian League would even hold together. And indeed, right in the middle of the Peloponnesian War (around 420-418 BC), Sparta faced a critical challenge to its supremacy when its truce with Argos expired.

The first years of the war demonstrated exactly how powerless Sparta really was. All it could do against Athens was invade with its massive coalition army in the hope that the Athenians would be stupid enough to come out and fight them in the open. On the advice of Perikles, they didn't (and even after Perikles' death, they never did). This made Sparta's one great trump card into nothing but a pointless show of strength. They'd march in, trample some crops, burn a few farms, but then they'd be forced to go home because most of their army consisted of Peloponnesian and Boiotian farmer militias that needed to tend to their own harvests urgently. Meanwhile the Athenians sailed all over the Aegean, around the Peloponnese, into the Corinthian Gulf and beyond - attacking towns, ravaging lands, easily destroying inexperienced Peloponnesian fleets, and subjecting numerous Spartan allies in Western Greece to their rule.

In this sense, Perikles' leadership actually held them back. His policy was to be cautious, to defend the status quo, and to avoid further conquests. His death allowed more ambitious leaders like Demosthenes and Kleon to take charge, leading to some of Athens' greatest victories during the war - including the capture of about 200 Spartiate prisoners on Sphakteria in 425 BC. Unfortunately, their campaigns came with high risks as well as high rewards, and a couple of major disasters (at Delion in 424 BC and Amphipolis in 422 BC) finally brought Athens to the negotiating table. This was also due to the more enterprising Spartan expedition to Thrace, which had actually managed to find an Athenian weak spot in its reliance on timber and precious metals from the region.

In the ensuing peace treaty (the Peace of Nikias, 421 BC), Athens traded some gains for recent losses, but maintained the bases it had established around the Peloponnese like daggers aimed at the heart of Sparta. It also retained its empire and naval supremacy. In short, it was no weaker than it had been when the war began (except for the loss of nearly a third of its population to the plague). Sparta had achieved not a single one of its war aims. Indeed, it had been practically begging for a truce since 425 in the hope of getting its captured citizens back alive. Sparta was humiliated in the face of its subject allies. This was a large part of the reason why Argos - neutral up to this point - declared war on Sparta almost immediately after.

No doubt the Athenian plan to sail against Sicily was hatched in part out of sheer confidence in their own invincibility after this result. It was only the loss of half the fleet and thousands of experienced warriors in Sicily that brought them down to a level Sparta had any hope of beating - if they could find a source of money comparable to Athens' tribute-paying subject allies. Once they found that source in the Persians, Sparta finally had a shot (though they would still lose their fleet with all hands, twice, before they finally crushed the Athenian navy at Aigospotamoi in 405 BC).