Sparta, including Messenia, was already the largest Greek city-state in terms of geography. Spartan society was based upon controlling their own helots, in Laconia and Messenia, who outnumbered the Spartans by 7-to-1. Even at the Battle of Plataea, Sparta left 3,000 soldiers behind in Sparta to watch over the helots and keep them from revolting and sent 5,000 Spartiates with loyal helots to the battle. So, the Spartan Army did not like to be absent from Sparta for long periods. The Spartans banned silver and gold and were afraid that they would be corrupted if they stayed too long among other societies. They did not have the same incentive to take gold and silver booty from enemies. And in fact, when Sparta stationed troops overseas, those officers or troops were sometimes corrupted by gold, silver and luxurious living. Sparta was not inclined or able to induce others to adopt strict Spartan ways. The number of full Spartiates declined after the Battle of Plataea, so that by the time the Spartans were defeated at the Battle of Leuctra, there were only 1,000 Spartiates.
The Spartans were a very exclusive society, not inclined to increase their numbers easily. This cost them any chance to expand into a great empire. In The Decline and Fall of the Roman Empire, Edward Gibbon wrote: “The narrow policy of preserving, without any foreign mixture, the pure blood of the ancient citizens, had checked the fortune, and hastened the ruin, of Athens and Sparta. The aspiring genius of Rome sacrificed vanity to ambition, and deemed it more prudent, as well as honourable, to adopt virtue and merit for her own wheresoever they were found, among slaves or strangers, enemies or barbarians.”