Roughly halfway through the Peloponnesian War, Athens and Sparta concluded a "peace treaty" due to the capture of a number of Spartiates by Athens in an earlier action and campaigns by Sparta against Athenian-dominated cities in Macedonia and Thrace. However, both sides continued to maneuver for domination, influenced by the possible power of the city Argos, which was located in the Peloponnesian peninsula but had traditionally been hostile to Sparta. Athens signed an alliance with Argos and some of their allies, and together they moved to conduct operations in the Peloponnesian peninsula against enemies of Argos and her allies. The Spartans sent forces led by King Agis against the allies twice, but the first time he was persuaded to leave without fighting despite having a superior situation, which led to harsh criticism in Sparta. Nevertheless, he was sent out again, whereupon he engaged and defeated the allies and broke up the Argos-Athens alliance, ending any chance of Athens neutralizing Sparta on land.
It struck me on reading the description of the battle given by Thucydides that the situation faced by both sides was very similar in some respects to the later Battle of Leuctra, where Thebes famously smashed the dominance of Sparta. In particular, in both battles the elite wing of the non-Spartan army was able to smash the forces in front of it, opening a chance to force the disintegration of the Spartan line (here, the right wing of the allied army, which was composed of the Mantinean and elite Argive forces, destroyed the Spartan left, composed of the Scritae, a non-Spartan people traditionally assigned to that side). However, whereas at Leuctra this caused the disintegration of the Spartan formation, at Mantinea the allies, instead, were beaten; while their elite was defeating the Scritae, their less elite forces to their left were fleeing from the Spartans on the shock of facing the relatively elite Spartan center, under the direct command of King Agis, with some of them fleeing even before entering battle (according to Thucydides). Thus, the victory of the allied right ended up benefiting them not at all, because they were flanked from the left and ended up fleeing along with most of the rest of their army. In the end, although the Spartans took moderate losses, the allies took much heavier casualties.
Hence, it is tempting to ask what would have happened if Mantinea had ended up being like the later Leuctra, whether because the allied contingents stand and fight rather than fleeing, or because of novelties in strategy from the allied command like those adopted at Leuctra. The army amassed by Sparta at Mantinea was very large by the standards of the time, and if it had taken substantial losses it could have permanently weakened Sparta and broken their hegemony over the peninsula. Additionally, the capture of Mantinea would have greatly weakened Sparta by blocking their main route out of the peninsula, thus isolating them from all of their allies. Together, these seem like they would have led to a decisive Athenian victory in the overall war, though I suspect that, like Sparta herself or Thebes later, the Athenians would eventually have destroyed themselves (perhaps even in the decisive Sicilian Expedition, in this timeline regarded as a separate though related conflict).
Thoughts?