The Peloponnesian War is always written about as the war(s) between Athens and Sparta. Yet when I read about the war(s) it always strikes me that Sparta is a reluctant protagonist, and that the core opponent of Athens are the Corinthians. So whilst Sparta is the most powerful Athenian opponent, Corinth strikes me as the most enduring, and for some very practical reasons. Competition over trade, colonies and that they both shared a border with Megara.

Some examples;
In ther Persian Wars, Eurybiades (a Spartan) is appointed Navarch over Thermistocles since the Corinthians won't submit to an Athenian. Eurybiades is mostly a figurehead.

The First Peloponnesain War (459-445BC) is initiated by a conflict between Corinth and Megara, and the Megarans conclude a treaty with Athens. 15 years war ensues.

The Peloponnesian War (431-404BC) is immediately preceeded by the Corcyran confrontation (and Battle of Sybota) in 433BC between the Corinthians and the Athenian backed Corcyans.

During the Peace of Nicias, the Corithians never recognise or accept the peace, and persuade the Boetaeans to do likewise. Moreover, the Corinthians support Argive aspirations in the Peloponnese to overshadow Sparta, apparently with the purpose of dragging Sparta back into the conflict.

Corinthian naval intervention against the Athenian Sicilian expedition.

At the final surrender of Athens in 404, Corinth and Thebes demanded Athens be destroyed and the Athenians enslaved, but the Spartans choose otherwise.

There seems to be to be a long history of emnity between Athens and Corinth, far greater and more enduring than that between Athens and Sparta. Corinth works in the background in the classical and contemporary histories, but is that simply the perceptions of Thucydides (an Athenian) and Xenophon (probably Athenian born, but a Spartan by residence)? Had a Corinthian written a history, would it show a different perspective?

Mark, I couldn't agree with you more on the role of Corinth as instigator in the conflicts between Sparta and Athens. Another item that you can add to your list of indicators is the vicious conclusion of the second battle at Cimolia in 458 B.C., when the Athenians under Myronides masacred a "considerable division" of the Corinthian army, trapping and stoning the retreating hoplites to death in violation of accepted military protocol (Thucydides 1,106). I believe that this incident, more than any other, helped to raise the Corinthians' realtionship with Athens from one of fierce economic competition into a state of intensely visceral hatred (especially among those many whose relatives died in the atrocity). Also worth adding to your list are the events of 432 B.C. at Potidaea, where Corinth sought to get back at Athens for its intervention at Corcyra earlier that year, ultimately setting off a revolt in that former Corinthian colony which was now an Athenian ally. This resulted in a significant land battle near that city in which the Athenians ultimately (and with some difficulty) defeated an army of Corinthian citizens, mercenaries and local men.

The engagement at Potidaea plus the naval action off Sybota show that Corinth and Athens were effectively at war on both land and sea long before the Spartans were pulled into the conflict. To judge by the Spartan king Archidamus' words of caution at the time (Thucydides 1.79-85), entry into the war was rather reluctant on the part of many in Sparta; certainly, on practical considerations and likely due to several past favorable relationships with Athens as well. I think that the Spartans' decision to join the fight probably boiled down in the end to their desire to keep the 'Peloponnesian League' alliance together, which meant giving in to Corinth. Unlike Athens, Sparta didn't have its own large and diverse military, but rather depended on allies to raise a fleet and sufficiently large army. Dominating the alliance's seapower and having its second most powerful army (behind only that of Sparta itself), Corinth had the leverage to get what it wanted from the Spartans. Would the Corinthians have actually brought down the 'League' to spite Sparta? Perhaps. In fact, after Sparta failed to go along with demands to destroy Athens in 404 B.C., Corinth did indeed refuse to participate in two subsequent Spartan campaigns: to quell what would prove to be a successful resistance at Athens in 403 B.C. and a punitive expedition against Elis in 402 B.C. It seems that hell hath no fury like a Corinthian ally scorned!

Mike, my earlier reply to your discussion seems to have gone astray, so I'll try again. I very much agree with your view that Corinth was a major instigator of the conflict between Athens and Sparta. Leading seapowers, Corinth and Athens were fierce economic rivals, which ultimately led to a very toxic relationship. Coming to blows in the 'Saronic War' (a phase of the 'First Pelopnnesian War' conflicts), their rivalry became particularly bitter after many Corinthian hoplites were slaughtered by Athens during their attempted retreat from the second battle of Cimolia in 458 B.C. In additon to the the clashes between these poleis that you've very accurately listed, there is another at Potidaea in 432 B.C. Responding to the interference of Athens at Corcyra earlier in the year, Corinth aided the city of Potidaea, a former Corinthian colony in the Grecian northeast, to revolt against Athens, it's current ally. This resulted in a phalanx battle that saw the Athenians defeat a mixed force of Corinthian citizens, mercenaries and Potidaean rebels. Thus, Corinth and Athens were already at war on both sea (Sybota) and land (Potidaea) well before Sparta was pulled into the affair.

Based on the warnings of Sparta's king Archidamus, many in his country seem to have been reluctant to declare war on Athens. This had both the many practical reasons for caution cited by the king as well as a cultural basis in famous episodes of Spartan/Athenian cooperation in the past. However, the Corinthians were in a positon to force Sparta to meet their demands for war. Unlike Athens, Sparta lacked a large and diverse military and was dependent on its allies in the 'Peloponnesian League' for both a navy and sufficient manpower for extensive land operations. Corinth had by far the League's largest fleet plus its second most powerful army (after only that of Sparta itself). Thus, if the Spartans wanted to keep the League strong and secure their homeland, it had to appease Corinth. Would the Corinthians have scuttled the Peloponnesian alliance in an act of spite over a Spartan refusal? Perhaps. In fact, after the Spartans failed to destroy Athens in 404 B.C., Corinth would not take part in two subsequent League campaigns: in 403 B.C. against an ultimately successful resistance movement in Athens and in 402 B.C. for a punitive expedition against Elis that ended short of full success. It seems that hell hath no fury like a Corinthian scorned!