What were the causes of the Peloponnesian War?

A bust of Pericles

One of the most important wars in the Ancient World was the Peloponnesian War (431-404 BCE). This was a long drawn out war between Athens and Sparta and their respective allies. It convulsed Greece and changed the course of the Classical world. The war ended the Golden Age of Athenian Culture and arguably weakened the Greek world forever. The origins of such a conflict are complex and these will be evaluated in this work. It will be argued in this piece that the ultimate origins of the war lay in Sparta’s fear of the growing might of the Athenian Empire.

In the aftermath of the Persian Wars, the two powers could not come to an agreement on their respective spheres of influence and this led to friction and eventually outright war. Athens and its ambitions also led to increasing instability and this is exemplified in the Megarian Decree. The profoundly different societies of Athens and Sparta was also a significant factor in the war’s outbreak, which also had an ideological aspect.


Statuette of a Spartan Warrior

The origins of the Peloponnesian War lay in the Greeks victory over the Persians. The Greeks had combined under the leadership of Sparta and Athens to defeat the Persians, then the most powerful empire in Asia. In the aftermath of the Persian Wars, the Greeks were unable to maintain their unity. The Greek world was riven by cultural and ethnic differences and people’s first loyalty was to their Polis or local city or community. There was a definite sense of ‘Greekness’ and a common cultural heritage.[1] Yet this was not enough to overcome deep divisions in the Greek world and as soon as the Persians left they immediately began to fall out amongst themselves. Sparta a very conservative society had opted out of the war with Persia once their invasion had been defeated. Athens continued the war against Persia and it formed the Delian League. This League was an alliance of city-states and islands that vowed to continue the war against the Persians.[2]

Over time the Athenians, who were the largest maritime power in the Aegean came to dominate the Delian League. This was the Golden Age of Athens and was the era of Plato, Socrates and countless other great cultural figures and artists. Gradually the Athenians began to turn the Delian League into an Empire. Athens used its superior navy to intimidate its allies and they eventually became mere tributaries of the Athenians. Sparta soon became very suspicious of the growing power of Athens. It was the head of the powerful Peloponnesian League, which comprised large city-states such as Corinth and Thebes. The League became very concerned about the Athenians huge fleet of ships that allowed it to dominate its former allies. Athens had also been turned into a formidable stronghold, by the Athenians, when they constructed the ‘Long Walls’. These walls connected the city with its port, Piraeus and allowed the city to supply itself and made any siege of the city unlikely to succeed.<re>Kagan, p. 113</ref> Athens growing ambitions led to tensions with its neighbors and eventually this led to a war. This involved Athens and Corinth, with the latter receiving some support from Sparta. This conflict ended with a peace treaty and a ‘Thirty Years Peace’, This in theory guaranteed Athens and Sparta their respective spheres of influence. Corinth and other members of the Peloponnesian League were unhappy about Sparta’s lack of leadership. Some leading Sparta became concerned that inaction would push the other major Greek powers, to side with Athens. During the so-called thirty Year Peace Athens grew ever stronger and in many ways arrogant, as seen in its increasing haughty attitude to its subject city-states.

Long-term Factors in the Outbreak of War

The underlying cause of the war was the rapid rise of the Athenians. They had grown from just another city-state to an Empire. During and after the Persian Wars, it had transformed itself and became a major trading and maritime power.[3] It had developed into the greatest maritime power in the Greek world and had the ability to dominate the trade routes in the eastern Mediterranean. It had emerged as a great Empire in a very quick period of time and this upset the traditional balance of power. For many decades’ Sparta, had been the greatest power in Greece, this was based on its well-disciplined and much-feared army. The Spartan Hoplite were considered the best soldiers in the Greek world.[4]

The rise of Athens meant that there were two great powers in the Greek world. These powers both had a network of alliances all over the Greek world and beyond. The Greeks became divided into a Spartan and an Athenian camp.[5] Athens and Sparta had different spheres of influences, as outlined in the ‘Thirty Year Peace’ treaty and theoretically this meant that they both could have lived in peaceful co-existence. Athens controlled the coastal areas of Greece and the Greek islands, while Sparta, a land power could control the Peloponnese. Despite this, Sparta grew increasingly fearful of Athens and its main ally Corinth was actively encouraging it to attack Athens.[6] In 440 BCE, Corinth urged the Spartans to wage war on Athens when it was suppressing a revolt on the island of Samos. The Spartan Kings always cautious had decided not to become involved in a war with their former allies. However, the Thirty Years Peace was under increasing strain. In the Spartan assembly, there was growing alarm at the growth of Athenian power.[7] As Athens seemed to be growing more powerful there was a growing war-party in Sparta. They argued that the Spartans had to attack Athens before it became too powerful. The fear of Athens increasingly led the Spartans to prepare for war, even though there is no evidence that the Athenians had any designs on Sparta or its allies.[8] There were those in Athens who believed that a war should be welcomed. There was a strong ‘imperial’ party in the city who believed that it was entitled to a great empire because of its role in the defeat of the Persians.

The belief that Spartan fear of Athens was the ultimate cause of the war was the view of Thucydides. According to, the great Greek historian, Thucydides the growth of the ‘power of Athens, and the alarm which this inspired in Lacedaemon (Sparta), made war inevitable.[9] "He believed that the Peloponnesian War was inevitable, because when a rising power confronted another power, they would inevitably wage a war against each other to further or protect their interests[10]. Some later historians have also argued that war was inevitable between the two greatest Greek powers. It is still widely held that in international relations, the growth of a nation-state or empire will inevitably lead to rivalry and war with an established power.[11]

Sparta and Athens

The Spartans and the Athenians were very different societies. Athens was a democracy, and it was very individualistic. The population played a very important role in politics and indeed it was a radical democracy. The citizens, (only free males) could directly vote on the affairs of the city. Sparta was almost the opposite of Athens in every way. It was a very stratified and conservative society. It was ostensibly ruled by two kings, from two royal families.[12] The kings shared power with a council of elders (Gerousia). Sparta society depended on a servile population the helots who toiled the lands of Lacodemia for their Spartan masters.[13] Sparta was a highly military society and the need for a strong and well-disciplined army was the main concern of the state. The state took boys from their family and trained them from youth to be soldiers. The role of women was to produce good soldiers and men were expected to be brave warriors.

The profound cultural and political differences between the two great Greek powers contributed to the war. They had real difficulties understanding each other and this lead to mutual suspicions. Because of their different political systems and cultures they were often ideologically opposed. Sparta favored the many oligarchies and distrusted the role of the common people in government. In contrast, Athens encouraged democracy and believed that it was the best form of government.[14] This ideological rivalry between Sparta and Athens did much to increase tensions in the run up to the outbreak of the Peloponnesian War and was a contributing factor.

Immediate causes of the War

Vase showing an Athenian ship, a Trireme

The tensions between the Athenians and the Greek only grew. Thucydides noted that many believed that war was only a matter of time and that the Thirty Years Peace Treaty would soon be broken. There was a crisis when a Greek colony in Thrace that was in dispute with Athens asked the Spartans for assistance. Athens despite this laid siege to the colony. The Athenians, at this time were also in dispute with the small city-state of Megara and they unilaterally banned the ships of that city from its port and its allies. This became known as the Megarian Decree. Megara was a long-time Spartan ally and this was widely resented, as it was seen as an attempt to make Megara completely dependent on Athens.[15] This was not acceptable to Sparta and they believed that if Megara came within the orbit of the Athenians that they would use the port to weaken their position in Greece.

Sparta, supported by her allies demanded that Athens withdraw the Megarian decree, but this was opposed by Athens. Pericles the de-facto leader of the Athenian Empire argued against such a move as it would only encourage the Spartans to make more demands.[16] Thucydides states that the Corinthians condemned Sparta's lack of action until then and warned them that they had remained too passive for too long. They demanded action. Sparta was concerned that if it displayed any weakness that this could lead to its losing its pre-eminent position in the Peloponnese League.[17]

The Athenians were extremely confident and they knew that as long as they had their navy and their ‘Long Walls’ that they could not be defeated even if they could not beat Sparta and her allies on the battlefield. This strategy was recommended by Pericles to the Athenians and was much praised by Thucydides. Sparta began to contemplate war but they seemed unwilling to formally declare war. Then the situation spun out of control when allies of Sparta attacked the allies of Athens. The Spartans came to believe that they had no choice but to go to war. In 431 BCE, the senior Spartan king led an army into the countryside around Athens and laid it waste. This was the start of the great Peloponnesian War. The early years of the war were a stalemate because according to Thucydides writings, this was because the Athenians followed Pericles cautious strategy.[18] Later Athens, encouraged by Alcibiades launched the Sicilian expedition to conquer Sicily during a lull in the fighting, known as the Peace of Nicias. This proved to be a disaster and it led to the loss of an Athenian army and navy. Remarkably, the Athenians continued to fight and the Spartans needed Persian help to defeat them.[19] The destruction of the Athenian fleet at Aegospotami ended the war, and Athens surrendered the following year. Athens was forced to tear down its Long Walls and was fortunate not to be utterly destroyed.


The Peloponnesian War changed Greece in every way. Nothing was the same after the war and Athens was never to be as powerful. The causes of the war, are that the Athenian Empire upset the balance of power in the Greek world. This greatly alarmed Sparta and its allies. The aggressive policies of Athens did not help the situation- the ambitions of the city-state certainly provoked the Spartans. Increasingly, the Spartans became very nervous about the growing naval and commercial power of Athens. At first, they resisted the calls of its allied to declare war on its arch-rival. Once Athens had issued the Megarian degree, it initiated a chain of event that led to the Spartan invasion of Athenian territory. The deep cultural differences between the two Greek powers was also a contributory factor to the increasing tensions that later exploded into an all-out war that consumed the entire Greek world.


  1. Kagan, Donald. The Outbreak of the Peloponnesian War (Ithaca, NY: Cornell University Press, 1989), p. 56
  2. Kagan, p. 113
  3. Cawkwell, George. Thucydides and the Peloponnesian War (London: Routledge, 1997), p 67
  4. Hanson, Victor Davis. A War Like No Other: How the Athenians and Spartans Fought the Peloponnesian War. (New York: Random House, 2005), p. 56
  5. Cawkwell, p.115
  6. Hanson, p. 117
  7. Kagan, p. 134
  8. Kagan, p. 213
  9. Thucydides, The Peloponnesian War 1.67–71
  10. Thucydides, The Peloponnesian War 1.67–71
  11. Kagan, p. 71
  12. Thucydides, The Peloponnesian War 1.67–71
  13. Thucydides, The Peloponnesian War 1.67–71
  14. Thucydides, The Peloponnesian War 1.67–71
  15. Thucydides, The Peloponnesian War 1.71-73
  16. Kagan, p. 115
  17. Thucydides, The Peloponnesian War 1.67–71
  18. Thucydides, The Peloponnesian War 2.69–71
  19. Thucydides, The Peloponnesian War 6.6–11


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