What was the population of the Delian League and the Peloponnesian League? Additionally, what advantages as well as disadvantages did both sides possess on the onset of the war?

##Population

For the situation in Greece at the beginning of the Peloponnesian War, we mostly rely on a single source: the Athenian Thucydides, who set out to write a history of the war (though he never finished it). He helpfully sets out Athenian military strength at the outset:

They had an army of 13,000 hoplites, besides 16,000 more in the garrisons and on home duty at Athens. This was at first the number of men on guard in the event of an invasion: it was composed of the oldest and youngest levies and the resident aliens who had heavy armour. (...) Perikles also showed them that they had 1,200 horse including mounted archers, with 1,600 archers unmounted, and 300 triremes fit for service.

-- Thuc. 2.13.7-8

This list does not include the number of citizens who were too poor to fight as hoplites or cavalry, and consequently fought as light infantry or as rowers in the fleet; these men are rarely numbered in Thucydides. Even so, the passage presents some of the best demographic information we have for any Classical Greek city-state. Using model demographic tables for pre-industrial societies, scholars have extrapolated from these figures a total of about 60,000 adult male Athenian citizens, and a total population of about 300,000 for the whole of Attika (of which perhaps 100,000 were slaves and 50,000 were resident foreigners).

We don't have comparable figures for any other part of the Athenian Empire. High modern estimates (gathered in J. Ober's The Rise and Fall of Classical Greece (2015)) put the population of Greece in this period at about 2.5 million, and the population of the greater Greek world at 4 million or so. Given the share which had fallen under Athenian sway, including populated regions like the coast of Asia Minor, it would certainly be fair to say that the population of the Athenian Empire was in excess of 1 million, though by how much is a matter of guesswork.

We are much less well-informed about the population of Sparta. This is in part because Sparta deliberately kept its citizen numbers secret, and Thucydides could not find out the truth (5.68.1). We know that they had a citizen body of about 8,000 adult males in the early 5th century BC, but we also know that this number declined steadily throughout the Classical period. The first data point we get is Thucydides' estimate that the Spartans fielded some 3,500 hoplites at the battle of Mantineia in 418 BC. Historians have juggled these numbers in different ways; estimates of the Spartan citizen population at this time vary between about 2,500 and well over 5,000. Given the numbers we hear of later, at the dusk of the Spartan empire, the former (lower) figure is likely to be closer to the mark.

These few thousand full citizens, however, ruled over a large population of non-citizen freeborn (called perioikoi) and state serfs (called helots). Since the Spartans controlled both their own substantial region of Lakonia and the neighbouring region of Messene, modern estimates put the total population of their domain at around 250,000. The vast majority of these were unfree.

As to the population of the Peloponnesian League, we have no idea. The territory it covered was much more modest than that of the Athenian Empire, although it spread beyond the Peloponnese to include the Megarid, Boiotia, Trachis and parts of Western Greece. The populous Peloponnesian city of Argos was not part of the League. As a result, the total population of this alliance system would have been rather less than that of the Athenian Empire. On the other hand, while Athens did not generally rely on military levies from its subordinate allies, preferring to exact tribute to pay for its own armies and fleets, Spartan military power was entirely based on mobilising allied manpower to serve Sparta's interests. According to Plutarch, the Peloponnesian army that took the field against Athens in 431 BC consisted of as many as 60,000 hoplites.

##Advantages/Disadvantages

From this overview, the main advantage of the Peloponnesian League is clear: it was able to field a far superior land army, led by the acknowledged masters of Greek land warfare, the Spartans. In addition, as a land-based and mostly agricultural power, the League was mostly self-reliant, and therefore not very vulnerable to blockading or threats to trade.

The main weakness of the League was that it had no money. The Spartans did not install a regular system of tribute paid into a League treasury; instead, when money was needed, they asked for one-off "contributions" from the allies. The result was that the more expensive types of warfare needed to break Athens - naval and siege warfare in particular - were largely beyond the powers of the Peloponnesian League. They were only able to engage in such activities on a modest and temporary scale until they were able to secure more funding.

The Athenians had the great advantage of thalassocracy - they ruled the sea. They could not be harmed by invasions of their territory, since their city was connected by Long Walls to its harbour and grain could be brought in by sea from trade partners on the Black Sea coast. In addition, their fleet allowed them to project force wherever they liked, invade any place that seemed exposed, and retreat just as quickly.

Their disadvantage, however, was that their own hinterland was effectively indefensible against the enemies that surrounded them on land; they had no way to fight the massive land army of the Peloponnesian League, and were forced to retreat behind their city walls whenever it was in the field. In addition, their reliance on grain imports, while a great advantage if the Spartans decided to wage the war on land only, made Athens terribly vulnerable to any spirited Peloponnesian campaign to take back control of the sea. If the Spartans could beat the Athenian fleet, Athens would almost instantly be brought to its knees.