Sparta couldn't even conquer more than their own backyard. They'd overrun Messenia and enslaved the lot, then spent a few centuries bickering with the rest of Greece, until Persia financed them to claim hegemony. That hegemony lasted 33 years, then they bickered with Thebes for a while, took losses, never quite recovered and eventually got subjugated first by the Macedonians, then by the Romans.