Was it a decline in birthing rates? A large defeat? Were they overwhelmed by the rise of the Roman Republic? I love ancient history and studied the Spartan agoge at school, but their decline is something I've never really gotten around to looking into. Any help much appreciated.
edit: Some great answers here. Thanks guys!
Edit: This is concerned with Sparta's immediate fall from Hegemonic power in the early 4th century BCE. For the continual decline into the Roman era, please see /u/edXcitizen87539319's terrific reply below!
Okay, so it all starts in 404 BC at the end of the Peloponnesian War. Sparta has finally beaten Athens after nearly three decades of conflict. At this point, power in Greece was solidly in the hands of Sparta and her allies. However, Sparta ruled as a hegemonic power, using her military strength to bully other poleis into submission. This continued until the 370's when they royally pissed off the Thebans.
Thebes had been under the Spartan yoke for quite some time, and eventually, they had had enough. A band of exiled nobles under the leaders Pelopidas and Epaminondas forcibly expelled the Spartan garrison from Thebes' acropolis, freeing the city. What comes next is one of the greatest upsets in military history.
These two leaders then organized the Boeotians (territory surrounding and including the city of Thebes) into a formidable army. It started with The Sacred Band, a group of 300 crack troops, of 150 homosexual pairs. The idea was that you'd be more ferocious in battle should your lover be standing right beside you. Originally founded in 378 BC by Gorgidas, and dispersed throughout the hoplite army of Thebes, when it fell under the personal leadership of Pelopidas, he kept the 300 as a cohesive and singular unit forming the spearhead of the Theban phalanx. This cohort was undefeated in battle until 338, when, at the Battle of Charonea, the 16 year old Alexander destroyed them to a man, but that's a story for a different day.
Epaminondas took over leadership of the entire Theban army, creating a phalanx far deeper than those before it. While previous phalanxes were maybe up to 12 ranks deep, Epaminondas' were up to 50! This took advantage of the fact that the Boeotians were mainly farmers, renowned for their hearty and robust physiques, but thought of as heyseeds: slow, agriculturally minded, militarily weak, peaceful, etc. This is where the Spartans went wrong. This Boeotian phalanx focused less on pure combat between the two lines, instead focusing on brute momentum from sixty rows of men shoving forward to destroy the opposing line. Man for man, the Spartans were the best soldiers the Greek world had ever known. That didn't matter when a hammer of powerful Thebans literally uprooted them from the ground.
After several years of open Theban revolt in the face of Spartan hegemony, Thebes had lost any allies she had due to Spartan threats, which finally manifested in a full-scale Spartan invasion. Opposing this large invasion force was the new Boeotian army, led by Epaminondas and Pelopidas' Sacred Band. At the village of Leuctra the two armies met. The Spartans, accompanied by their allies numbered around 10,000, the Thebans, 6,000 (According to Wikipedia). The Spartans had never before lost a land battle in which they outnumbered their opponent. Here, they did.
Now, typically, the best troops were placed on the right flank of a phalanx battle, the weakest on the left. This left the weakest to face the strongest, and preserved the cream of the crop. Epaminondas wasn't about this. In a departure from accepted custom, he placed the Sacred Band directly across from the Spartan elite, the hippeis. Then he had his other divisions slowly pull back from battle to save them (and Sparta's allies-whom he professed no quarrel with) as he crushed the Spartan main contingent. The Sacred Band destroyed the Spartan troops, killing one of their two kings, Cleombrotus.
The effect of this victory cannot be overstated. It ended the Spartan mystique and their military domination of the country. From here, the Theban generals led an attack straight to the Spartan mainland. Along the way, the Theban army drew thousands of Greeks who were keen on destroying Sparta forever. Victor Davis Hanson estimates that in total, 70,000 Greeks ended up marching on Sparta, somewhere upwards of 5% of the total Greek population at the time.
Sparta at this time had suffered from severe downward population pressure. Their continuous military involvement had drained them of much of their male population. Additionally, their "mating habits," if you will, nocturnal visits by husband to wife at an irregular frequency, paired with their infanticidal custom didn't help their numbers very much. It must also be remembered that the Spartan state relied on the enslaved region of Messenia to furnish them with food, slaves, and basic industrial goods. At this point, due to the diminishing population, the Spartans were even arming the Messenians-a scenario that the entire military state had been created to prevent! Seeing the weakened balance of power in Laconia (Sparta and the area of southern Greece under her direct control), Epaminondas besieged the city, but, instead of sacking it, turned to Messenia and freed the helots.
The helotry of Messenia outnumbered the Spartan citizens by perhaps more than 2:1. The Thebans armed the Messenians, built fortified cities, and forever freed them from the Spartan yoke. This destruction of the Spartan status quo, forever prohibited the Spartans from focusing their entire society on the creation of an exemplary military force, instead relegating them to the same sort of societal structure shared by the other Greek poleis.
Sorry if I missed things, please ASK QUESTIONS! I figured I should end there, but I'd be happy to talk more about this. For further reading, I'd suggest:
Xenophon's Anabasis 6.5
Diodorus Siculus, Chapters XV-XVI
Plutarch's Life of Pelopidas
Victor Davis Hanson's The Soul of Battle
This answer is based in large part on an earlier answer I've written. I have fleshed out the early parts a bit but the part from Rome's entry to Greece onwards is largely unchanged (the previous question was particularly on Sparta under Rome).
Edit: I don't really do the 'early parts' justice (it is before my time, so to speak). Read the excellent reply by /u/kosherkowboy for the early decline of Sparta, then come back here to see what happened later on!
Sparta's height of power was in the 5th century BC. During the 4th century Sparta gradually lost power: important city states such as Athens, Thebes, and Corinth allied against Sparta (the Corinthian War); the Persians took back Spartan holdings overseas (in Asia Minor) and shortly after the Corinthian War Thebes dealt a major blow to Sparta's power by defeating them in battle.
This is when Sparta's gradual decline began in earnest. For a while, Sparta remained a proud and independent city-state, but it wasn't really a major player any more. Sparta was the only Greek city not siding with (or forced to side with) Alexander the Great in his campaign against the Persians in the late 4th century. It was just capable enough to withstand Macedonian pressure, but it wasn't nearly powerful enough to be a threat to Alexander or his Greek allies (indeed Sparta had lost another major battle against Greek forces only a few years earlier).
For more than a century Sparta remained in this state of being past-their-prime-but-not-yet-irrelevant. It is then that we see Roman involvement in Greece starting. During the First Macedonian War (late 3rd century BC) Rome sought the help of the Aetolian League, granting the rights of amicitia (friendship) to the Aetolian League and any city state helping Rome in the war against Philip V of Macedon (Rome was at the same time fighting the Second Punic War, so it had limited resources available for the war in against Philip V). Sparta sided with Rome in this war, and so became an amicus of Rome.
Being an amicus was somewhat short of being an ally; it was certainly not a defensive pact and dit not guarantee any protection from Rome. So it happened that Sparta in 201 attacked Messene, another amicus of Rome, and in 198 - during the Second Macedonian War - Sparta made peace with Macedon even though Rome remained at war with them. Pragmatic as always, the Roman Senate revived the amicitia with Sparta the next year, under condition that Sparta would assist in the conflict.
At the end of the conflict, Rome threw Sparta's actions back in their face. Livy reports that the Romans said this to Nabis, the Spartan ruler:
Though you were our ally you seized by force a city in alliance with us, namely Messene, which had been admitted to our friendship and enjoyed precisely the same privileges as Lacedaemon [Sparta - ed.]. And further, you not only concluded an alliance with Philip, our enemy, but you actually established a relationship with him through Philocles, one of his viceroys. In open hostility to us, you infested the sea round Malea with your piratical barques, and have seized and put to death almost more Roman citizens than Philip, so that our transports, which were supplying our armies, found coasting along the Macedonian shores safer than rounding the Cape of Malea. Forbear henceforth, if you please, to talk about your loyal observance of treaties; drop the language of a citizen and speak as a tyrant and an enemy. (Livy 34.32)
As Rome had given many Greek cities their liberty by defeating Philip V of Macedon, so too would they give liberty to the city of Argos, which was under Spartan occupation. Under this pretext Sparta was attacked and defeated by Roman and Greek forces, ending whatever remained of the Spartan hegemony in Greece. Sparta was forced to join the Achaean League.
For the next fifty years or so Rome remained involved in Greece while proclaiming that the Greek cities were free. During the Third Macedonian War (171 – 168 BC) Rome took prominent members of Greek communities as hostages to Rome, to ensure the continued loyalty of the "free" Greek cities. Eventually, after the final defeat of Macedon in 148 and the incorporation of Macedon in the Roman empire, it came to war between Rome and the Achaean League. The catalyst, interestingly, was Sparta.
Since Sparta was forced into the league, it continuously tried to leave the league. At least three times the Spartans or Spartan exiles petitioned Rome with challenges tot he jurisdiction of the Achaean League, each petition being counter petitioned by representatives of the league. As Erich Gruen put it:
Senatorial reaction was cast in the traditional mold: evasive and noncomittal answers which each party read in accord to its own wishes and employed to justify steps already determined upon. (p. 520)
Finally Sparta seceded from the league, and the Achaean League responded with force. They figured this was an intra-Greek conflict; the Greeks were nominally free and Rome had so far proven to be mostly indifferent to what happened. Rome responded swiftly and decisively though. Roman forces moved from Macedonia and Italy into Greece and crushed the Achaean League. From then on Greek cities were no longer allowed to form confederations. Some cities - those which had resisted - were put under control of the Roman governer in Macedonia; others, including Sparta, remained "free". It would take another century before this charade was dropped and Greece was made into an official province of Rome. By then the Greek city-states had lost most of their relevance, except perhaps to serve as cultural guides (the Romans of that time loved Greek culture).
To answer your question:
Sparta went through a long proces of gradual decline. Over the course of centuries, it's status changed from being the most important power in Greece to just another Greek city without much relevance. The main reason for Sparta's decline was political (though there may have been some social aspects). Sparta was on the losing end of the Greek system of coalition warfare a few too many times.
As for the agoge (since you mentioned it), this reply in the other thread by /u/Azand may provide some insight.
Main source used:
Also interesting (on Roman expansion and its nature):
W.V. Harris, War and Imperialism in Republican Rome 327-70 BC (Oxford 1979)
A.M. Eckstein, Senate and General: Individual Decision-making and Roman Foreign Relations 264-194 BC (Berkeley 1987)
I wrote my undergraduate dissertation about the topic of Sparta's decline in power in the 4th century so thought I would share my bibliography on the topic as many of the articles and books below go into great detail about the topic. Firstly I will briefly post my concluding thoughts on the issue though (can go into more detail tomorrow if you want)
My conclusion was that it was the longer term issues facing Sparta such as the declining birth rates, inequality of wealth, the challenges of the helots which were exasperated by shorter term challenges and issues such as the challenges of managing a Greek Hegemony, Agesilaus reign and the rise of Thebes that led to Sparta's decline Edit - and to answer the question of when Sparta ceased to be a military power well it ceased to be the major military power in Greece in the aftermath of Leuktra however it still remained a military power for decades (my immediate area of study ends around 350 so after that I am sure others can elaborate on Sparta's military after this period in a better way)
edit - Sorry about formatting its acting really strange when I try and copy it over
Primary Sources – from http://www.perseus.tufts.edu/ (Search Sparta)
Secondary sources Books
