Sparta in the fourth century BC had once been immensely powerful. Before taking action, it had no need whatsoever to consult the weak city-states around it. The Athenians had once accused the Spartans: "You start wars whenever you please and never ask your allies for their opinions, yet demand that they follow you into battle. As a result, those so-called self-governing city-states are forced to turn their weapons against their own friends."
In truth, the Athenians were exaggerating somewhat, because the Spartans would only convene an allied council "when they needed to." For example, when launching a large-scale, highly dangerous war, or when the journey was long and the opponent powerful—when Sparta risked having no return if it ventured deep into enemy territory alone—that was when the Spartans needed their allies.
Other factors also played a crucial role. Sparta did not treat its allies equally. The first type of ally was weak and easily influenced by Sparta. The second type was somewhat stronger, or farther from Sparta, or both, but still not strong enough or distant enough to escape Spartan control. The third type of ally was either extremely powerful or very far from Sparta.
Corinth, for instance, straddled the isthmus. It could keep Sparta's enemies outside the Peloponnese, but it could also allow them into the Peloponnese and threaten Sparta's security.
Corinth's wealth was as famous as Sparta's poverty. For various reasons, Corinth's opinion could not be ignored. During councils, Sparta's policies could not be carried out simply because Corinth opposed them. Sometimes, Sparta's policies were even decided by Corinth.
Corinth had a neighboring state called Megara. Because of a territorial dispute, it went to war with Corinth. At first, the Spartans ignored the matter. Later, Megara broke away from Sparta and joined the Delian League led by Athens.