I understand that after 2500 years, the Spartan mythos would be highly exaggerated, but even so from my understanding the Spartan Army was a highly professional, highly trained, highly motivated force that faced an army that neither trained as often nor as hard as the Spartans.
When an Athenian claimed that 'We have often driven you from the Cephisus'. Antalcidas replied "But we have never driven you from the Eurotas."
How exactly did the Athenians manage to drive the Spartans away at all if the Spartan army was so superior?
To clarify: is your question about the strength of the Athenian army at a particular battle, or is it about Athenian military strength in general in the late 5th and early 4th centuries?
There are actually a couple of frequently asked questions based on the Spartan portion topic, but the sub always like to leave the door open for fresh contributions, so some Athenian POVs would probably be really cool. Here are some links:
If you're looking for a specific example rather than a general one, Thucydides provides a possible answer to your question through Perikles' speech in 1.140-144 of The Peloponnesian War. His main points are that the Peloponnesians, while they have a powerful army, are not wealthy enough to conduct extensive offensive campaigns, and that their naval power is far inferior to that of Athens. The Athenians are aware that they may be bested on land, but are confident that they can more than make up for this on the sea.
I'm not well versed in Greek history, but it's very important to note that Sparta and Athens didn't exist in political vacuums. Greek wars were wars of alliances, in which these particular states often found themselves opposed. popular culture often prescribes to much importance to these to polises in particular: there is no way that Sparta or Athens would ever be able to challenge a united Greece.
Someone will most likely correct me on this, but I also believe that Sparta was generally stronger in combat than Athens; Athens however was very difficult to siege because of the long walls connecting it to Piraeus and it's naval supremacy. Athens would also often be raiding the coasts of the Peloponnesian peninsula.
But I think the best answer to your question lies in the fact that there were no isolated conflicts between these two polises.
Okay if you can have ships, and you fight in an Island nation you can literally surround and cut off any army on land. Even if the Spartans are better in the actual battle, a phalanx can only face one way. If you are caught behind by reinforcements, you surrender. There were a lot of Spartan Prisoners of War (so much so that they started making serious concessions to get some of their troops back.)