The first 10 years of the War between Athens and Sparta are considered to have constituted a separate conflict from what is generally known as the Peloponnesian Wars. Named after the Spartan king Archidamos who presided over the assembly of Sparta and its allies at Sparta in 432 that voted for war.
The relative strengths of Athenian league and Peloponnesian league at the outbreak of hostilities in 431 were the strengths and weaknesses of a sea power versus a land power. Athens would always be vulnerable in Attica and Thrace were the laying waste of infrastructure and crops by an invading army would force the Athenians to fight a pitched battle. A battle that the Spartans had every reason to believe that they would win. If no such confrontation could be achieved then the very certainty and destruction of these attacks would weigh heavily on the Athenian population and force them to a settlement on terms advantageous to the Spartans and their allies. As long as the wooden walls (the Athenian navy) ruled the Aegean and the stone walls stood Athens could not be starved into submission, the Greeks not being strong in siege warfare, starvation was usually the usual strategy but this could take some time.
On the other hand the Peloponnesian coastline was vulnerable to raids by the powerful Athenian Navy. Pericles also doubted the ability of the Athenian citizen hoplite to defeat Sparta on land. His strategy was to retreat behind Athens strong walls and wait for the Peloponnesians to go home after a summer of ravaging Attica and strike back by using their naval forces to attack the territory of Sparta and her allies hoping to make them lose their enthusiasm for the war and bring them to the peace table.
Like all wars it was not expected to last long before one or the other would tire and a settlement would be reached but the very nature of both sides strengths and weaknesses ensured that unless a third party intervened on one side to tip the scales this war could and did drag on and inevitably the winner by accepting the consequences of foreign intervention would sow the seeds for its own downfall.
It has been estimated that the total number of Athenian men of military age was about 40000. Of these 1000 were wealthy enough to serve as cavalry. About 20,000 of the rest were eligible to serve as hoplites but only half of these would be called upon to serve at any onetime. In practice the Athenians mobilized their own citizens and their subject allies from the Delian League and supplemented by mercenaries in times of war. The breakdown of a force can be illustrated by the forces that were sent to Sicily in 415. Of the 5,100 Hoplites, 2,200 were Athenian citizens, 750 were mercenaries and 2,150 were from the Delian League. The fleet also required large numbers, a trireme normally required 150-170 oarsmen plus specialists and 'marines'. Crews were normally contracted for as few cities had the necessary resources to man a large fleet.
At its greatest extent the total number of Spartan citizens was never large, by 480 this may have been around 8,000 but by the start of the war this may have been only 5,000 citizens of military age available for service. In any Peloponnesian league army the bulk of the manpower was provided by their allies, the cities of Arcadia, Mantinea, Tegea, and Orchomenus maybe 2-3,000 hoplites each. Corinth could probably field 5-6,000 hoplites and Megara 2,000. Thebes as head of the Boeotians League could outfit 6-7,000 heavy infantry of their own plus 3-4,000 from the league.. The only other city that could put a substantial force into the field was Argos and she was 'neutral' in 431.
In May 431 Archidamus leads much of Peloponnesian army in an invasion of Attica. An offer of peace is sent ahead before formal hostilities commence but is rejected. The Athenians move their people and property within their walls and their livestock to the island of Euboea before Archidamus ravages the Eleusis and Thracian plain.There were 5 annual invasions between 431 and 425, they did not always last a long time now were they very effective. Athenian cavalry harassed them and Athens could import much of its required food via secure shipping routes. Also it was difficult to mobilize the Spartans allies for any length of time as they desired like all amateur soldiers to be at home working in their fields. The longest period of invasion in 430 lasted only 40 days.
Attacks on the coast of Argolid with 4,000 infantry and 300 horse, commanded by Pericles were made. in order possibly to capture the city of Epidaurus, whose troops were away with the Peloponnesian army. The reasons for this were twofold.
Before the war, the Amphilocians had been harassed by the Ambraciotes and they appealed to Athens for help, Phormio was sent with 30 ships and had restored the situation. Now he returned to Naupactos with 20 ships to monitor the situation and to guard the Corinthian gulf.
Early in 429, the Spartan general Cnemus managed to slip across into Acarnania with 1,000 hoplites to help an Ambraciot army attack the city of Stratus. The Corinthians sent out a fleet of 47 triremes to reinforce him, which was intercepted by Phormio's 20 ships. The Peloponnesians showed little confidence in their naval ability. Despite their numerical superiority, the Corinthians adopted a purely defensive formation. They eventually lost their formation and their ability to manoeuvre. Phormio attacked quickly and the Corinthian fleet was routed.
Thinking that the defeat was due to cowardice and not due to a basic weakness in tactical ability or lack of experience the Peloponnesian fleet was reinforced by 77 ships under the joint command of Brasidas and Cnemus. Initially successful the Peloponnesians became strung out and the leading ships (Leucadians) were counter attacked successfully by Phormio capturing 6 ships. Timokrates a Spartan commander on one of the captured vessels put death before dishonour and killed himself.
Learning that 40 Megaran ships were laid up in Nisaea, the Peloponnesian commanders came up with a cunning plan taking advantage of the fact that in their over confidence in their naval superiority the Athenians had left the harbour of Piraeus open and unguarded. Taking their crews across the Isthmus of Corinth, the Peloponnesians manned this new fleet and left Nisaea overnight with the chance if successful of ending the war with one stroke. However caution got the better of their nerve and they contented themselves with plundering the isle of Salamis. By morning the Athenian fleet was ready and the Peloponnesians withdrew.
In 429 instead of marching on Attica a third time, Archidamus moved across Mount Kithairon and laid siege to Plataea. An attempt to negotiate a surrender failed on Athenian assurances of aid, which ultimately did not materialize. The garrison at this time were 400 Plataeans and 80 Athenians. Archidamus invested the city with a palisade to prevent anyone escaping and built a mound against the southern wall. All attempts to force the walls ended in failure and thus the Spartans began a long blockade by surrounding the city with a rampart and digging ditches outside the walls and leaving a garrison to winter there. In the following year with no chance of succour from the Athenians about half the garrison succeeded in breaking out, the remainder succumbed to starvation and surrendered the next year. On the insistence of the Thebans they were treated harshly, the defenders executed and the non-combatants sold into slavery.
By the Autumn, of 427 the Athenians had blockaded the 2 harbours of Mytilene, and 1,000 hoplites were sent under Paches who arrived before the Spartans. In desperation the oligarchic regime at Mytilene had weapons distributed to the masses but this plan backfired as the newly empowered citizenry handed the city over to Paches who sends home for instructions. Cleon persuades the Athenian assembly to make an example of Mytilene for any others who may be thing of rebelling by executing all the men and enslave the women and children. To this end a ship is sent.
The next day a change of heart occurred and a second ship is sent out with revised orders. Fortunately for the citizens of Mytilene it is induced to great efforts by the promise of riches by the Mytilene envoys and arrives just in time. Most of the citizens were spared except the ring leaders and Mytilene was deprived of her fleet and much of her territory.
The war continued to descend into new depths of savagery as 427 drew to a close. A civil war broke out in Corcyra between pro-Athenian "democrats" and pro-Spartan "oligarchs." The Athenian fleet at Naupactus, now commanded by Nicostratus, soon arrived and arranged a truce favourable to the democratic faction, but before it could depart, Alcidas and the Peloponnesian fleet sailed up in support of the oligarchs. A confused and indecisive battle ensued. The arrival of an Athenian fleet of 60 ships under Eurymedon sealed the fate of the Corcyran oligarchs many of whom were massacred by Corcyran democrats.
In the summer of 426 the Athenians sent out two fleets, one under Demosthenes to cruise around the Peloponnese, the second under Nicias to attack Melos. Nicias' force of 60 ships and 2,000 hoplites failed to bring the Melians to terms, so he sailed north to raid the Boeotians coast. Demosthenes, meanwhile, reached Naupactus and embarked on an ill-fated invasion of Aetolia. He hoped that after conquering the Aetolians, he could raise an army to invade Boeotia. Unfortunately for this grand plan, his initial invasion ended in disaster. His small army made it to Aegitium, however, the Athenians found themselves surrounded by an army of lightly-armed troops, and got a harsh lesson in the vulnerability of a hoplite phalanx unsupported by light troops.
The Spartans, taking advantage of the hostility that Demosthenes' invasion had caused, agreed to send an army to join the Aetolians in attacking Naupactus. In the autumn of 426, 3,000 Peloponnesian hoplites commanded by Eurylochus assembled at Delphi and met up with their Aetolian allies in front of Naupactus, but found the city well-defended by 1,000 Acarnanian hoplites that Demosthenes had brought.
The arrival of Eurylochus's large army soon generated a new opportunity. Envoys from Ambracia met with Eurylochus and urged him to assist them in attacking Acarnania. The Spartan commander accepted the plan. The two armies joined up near Argos at Olpae and met an Acarnanian army led by Demosthenes. After a hard-fought battle, the Acarnanians drove the enemy back into their camp, killing Eurylochus. Demosthenes sealed his victory by destroying a second Ambraciot army which had been sent as reinforcements for the first on the next day.
In 425 an Athenian fleet of 40 ships under Eurymedon and Sophocles accompanied by Demosthenes is sent to Sicily at the invitation of the city of Leontini which had asked for aid against Syracuse. At the suggestion of Demosthenes an outpost is to be established at Pylos on the west coast of Messenia as a base to raid Laconia and stir up the Messenian helots to revolt against Sparta. With this in mind a small fort with garrison was left. This forced King Agis who was invading Attica to immediately withdraw his army back to the Peloponnese.
The Spartans put 420 hoplites drawn by lot under Epitadas on the island of Sphacteria which protected Pylos to prevent the Athenians from landing there, and planned to use their fleet of 60 ships to block the Athenians from entering the harbour. The garrison was attacked by land and sea for 2 days when the Athenian fleet returned and drove off the Peloponnesian ships cutting off the Spartans on the island. The situation was so desperate, no food or reinforcements could be landed nor could they be rescued, that a truce was arranged on terms that were very disadvantageous to the Spartans. Not only had they to surrender what remained of their fleet but also all other ships in Laconia while the truce lasted and sent ambassadors to Athens to negotiate the end of the war. The Spartans were prepared to make huge concessions but the Athenians insisted on a public humiliation. Thus the war party under Cleon won the day, the envoys returned home and the Spartans somewhat naively asked for their ships back. The Athenians refused claiming that the Spartans had not adhered to some of the details of the agreement.In the meantime the Spartans managed to bring in supplies in when high winds made it difficult for the Athenian ships to maintain their position. For the Athenians it had been a hard blockade, they themselves weren't well supplied, and water was short. As the Pylos blockade drags on Athenians at home begin to get impatient. and they start to think that they should have used the opportunity they had to end the war, that Cleon gave bad advice. Cleon responds that he could finish the blockade in 20 days if he were given command. Nicias offers the command leaving Cleon no option despite his lack of military experience but to accept taking. forces including archers and other light-armed troops.
Cleon arrives at night, forces put onto a few boats, land on the south side of the island before dawn, where there is a small Spartan outpost there are something like 10,000 Athenians and their allies consisting of 800 hoplites and 800 archers and various other light armed troops including sailors against 420 Spartans and maybe 1,000 helots but the Spartans have the defender's advantage, and the terrain is not favourable to the use of hoplite tactics. Demosthenes cleverly breaks his force up into 'combined arms teams' of 200 men. The Athenians overpower the southern outpost, light-armed troops advance along a high ridge on the harbour side of the island toward the main Spartan camp, and send another detachment along the low shore on the sea side, to squeeze in the Spartans. Spartans retreat to the high hill on the north of the island using the remains of an old prehistoric wall to make a defensive stand. At first they repelled the assailants and refused to surrender. As at Thermopylae the position was turned when a Messenian soldier tells the Athenians about a path to the rear of the Spartan position. A detachment takes a boat to a gorge that empties out to the east of the Spartan position, climbs up and takes the Spartans by surprise. Their position now untenable, their leaders dead or injured, the surprise to all Greece was that after consulting their superiors on the mainland and being given somewhat unhelpful advice they finally agreed to surrender. 292 of the original 420 were taken prisoner, of whom120 were Spartiates these were taken to Athens as hostages.
The action at Pylos provided Athens with two major advantages : firstly a military camp in the Peloponnese, and secondly prisoners for bargaining, which were security against further invasions of Attica for the Athenians had promised to execute them if there were further invasions. In 424 the Athenians formed an expedition of 60 ships and 2,000 hoplites and took the island of Cythera to the south of Laconia defeating the perioecic Cytherans and the garrison. They persuaded them to join the Athenian League, the alternative was death. From here they were free to raid the Laconian coast forcing Sparta to disperse their forces to defend their farm lands. Thyrea was the next target where the Spartans had settled the Aiginetan exiles, this to fell and the Aeginetan survivors executed. The Athenians then attacked Megara in support of a pro Athenian democratic faction. The Peloponnesian garrison at the port of Nisaia surrendered offering to hand over the Spartans there. Megara did not fall due to the resolute action of Brasidas who was at Sicyon recruiting. gathering a force of 4,000 hoplites and some Boeotian cavalry and checked the Athenian advance.
The plan was for Demosthenes to return to Naupaktos, gather a force of Acarnanians, secure Siphai, the port of Thespiai, while Hippocrates marched with the Athenian army to Boeotia on the northeast frontier of Attica, at Delium and instigate a revolt in Chaeronea. However the Boeotians got wind of the plan, occupied Siphai and Chaeronea, levied a large army to oppose Hippocrates at Delium. Demosthenes had to retire when he finds out Siphai has been guarded but Hippocrates was able to reach and fortify Delium with 7,000 hoplites and 20,000 light-armed troops. He built a palisade and trench within a few days and withdrew most of the army back to Attica.
Boeotians forces, which had been gathering at Tanagra a few miles from Delium, decided to attack the Athenian forces in retreat, were now on the other side of a hill that blocked the Athenian view. This force consisted of 7,000 hoplites, but also 1,000 cavalry and 10,000 light armed troops. The Thebans are on right of the Boeotians line, drawn up 25 deep; other Boeotians contingents are at various depths. The Athenians in a standard 8-deep formation, were getting ready for the attack, when Boeotians charged from the crest of the hill. Athenians held out on their right but on left, they cannot withstand the deep Theban phalanx. A squadron of Boeotians cavalry comes around the hill, and the Athenian army, thinking it's the vanguard of another Boeotians army, flee, Hippocrates and 1,000 Athenians are killed. The garrison at Delium is blockaded and is taken..
Thucydides describes it best:
"The Boeotians placed a detachment to deal with these, and when everything was arranged to their satisfaction appeared over the hill, and halted in the order which they had determined on, to the number of seven thousand heavy infantry, more than ten thousand light troops, one thousand horse, and five hundred targeteers.  On their right were the Thebans and those of their province, in the centre the Haliartians, Coronaeans, Copaeans, and the other people around the lake, and on the left the Thespians, Tanagraeans, and Orchomenians, the cavalry and the light troops being at the extremity of each wing. The Thebans formed twenty-five shields deep, the rest as they pleased.  Such was the strength and disposition of the Boeotians army. "
"Hippocrates had got half through the army with his exhortation, when the Boeotians, after a few more hasty words from Pagondas, struck up the paean, and came against them from the hill; the Athenians advancing to meet them, and closing at a run.  The extreme wing of neither army came into action, one like the other being stopped by the water-courses in the way; the rest engaged with the utmost obstinacy, shield against shield.  The Boeotians left, as far as the centre, was worsted by the Athenians. The Thespians in that part of the field suffered most severely. The troops alongside them having given way, they were surrounded in a narrow space and cut down fighting hand to hand; some of the Athenians also fell into confusion in surrounding the enemy and mistook and so killed each other.  In this part of the field the Boeotians were beaten, and retreated upon the troops still fighting; but the right, where the Thebans were, got the better of the Athenians and shoved them further and further back, though gradually at first.  It so happened also that Pagondas, seeing the distress of his left, had sent two squadrons of horse, where they could not be seen, round the hill, and their sudden appearance struck a panic into the victorious wing of the Athenians, who thought that it was another army coming against them.  At length in both parts of the field, disturbed by this panic, and with their line broken by the advancing Thebans, the whole Athenian army took to flight.  Some made for Delium and the sea, some for Oropus, others for Mount Parnes, or wherever they had hopes of safety,  pursued and cut down by the Boeotians, and in particular by the cavalry, composed partly of Boeotians and partly of Locrians, who had come up just as the rout began. Night however coming on to interrupt the pursuit, the mass of the fugitives escaped more easily than they would otherwise have done.  The next day the troops at Oropus and Delium returned home by sea, after leaving a garrison in the latter place, which they continued to hold notwithstanding the defeat."
The winter of 424-423: sees Brasidas using his base at Amphipolis to consolidate control of more of the Chalcidice, in particular the Acte peninsula, the eastern finger, and Sithonia, the central finger. He wins over towns in the region by treating them well and offering good terms.
In the wake of the defeat at Delium and the loss of Amphipolis, the Athenians are again interested in peace:
Meanwhile, Scione on the western prong of Chalcidice revolted and invited Brasidas in, surprising because of what happened to Potidaea: Scione is cut off by land and vulnerable by sea. The revolt happened two days after the armistice was concluded, but it's only after Brasidas is already in Scione that messengers from Athens and Sparta arrive to announce the truce. Shortly afterwards, the neighbouring town of Mende also revolted, and Brasidas doesn't hesitate to accept their alliance, even though now he knows of the truce between Athens and Sparta. Brasidas might even have made greater gains but he had to divert most of his forces to help Perdiccas, the Macedonian king (who was helping to finance Brasidas' operations). Taking advantage of his absence Nicias, with 50 ships, recovers Mende and is in the process of blockading Scione. Scione holds out longer but Spartan reinforcements are blocked in Thessaly as Perdiccas is now co-operating with the Athenians. Despite the truce the war was carried on in sporadic fashion in Boeotia where the Athenian fort of Panakton was captured making northern Attica more vulnerable to raids and the supply line from Euboea more tenuous.
After the expiration of the truce, in 422, Cleon carries a resolution in the Assembly that an expedition be made to recapture Amphipolis.
When he got to the region, he took some of the troops that were still blockading Scione, and went on to take Torone and capture the Spartan governor there. He retired to Eion, where he intended to wait for reinforcements from Thrace and Macedonia before attempting to retake Amphipolis.
Brasidas, since winning over Amphipolis two years earlier, had set about reinforcing the fortifications of the place, in particular with a wooden palisade to bring the bridge over the Strymon into a more defensible position. He also hired additional Thracian mercenaries.
When Brasidas find out Cleon is at Eion preparing for an attack on Amphipolis, he brings about 1,500 troops, out of a total force of 3,500 or so, out to Mt. Kerdylion so he could monitor the west, from which Macedonian reinforcements might arrive, and look toward Eion to monitor Athenian activity.Cleon moved his forces from Eion toward Amphipolis; not waiting for reinforcements as the forced inactivity was leading to discontent among the troops, who were probably not confident of Cleon's leadership. He takes up a strong position on a hill opposite the city. When scouts reported that Spartan forces within the city were preparing to come out he ordered his forces to withdraw.
Brasidas brought a force of 150 hoplites out of the gates and charged the Athenian centre, whose unguarded flank was exposed toward the city. The left wing, leading, took to flight, and at the same time, the rest of the Spartan troops under Klearidas issued forth from the Thracian gates and attacked the Athenian right, bringing up the rear. The Athenians panicked and fled. Cleon was killed by a Thracian mercenary along with 600 of his hoplites. Only 7 of the Spartan forces were killed but one was Brasidas who as usual was leading by example. He lived long enough to hear of his victory. The jubilant citizens of Amphipolis gave him a magnificent funeral and installed a shrine to him as city founder in place of the Athenian founder. So died the most un-Spartan of Spartans but the best of them.