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In the late 5th and early 4th centuries BC, the Spartiate class gradually declined, along with Spartan military prowess. There were several reasons for this decline. First was attrition through the increasingly frequent wars that Sparta found itself embroiled in from the mid 5th century on. Since Spartiates were required to marry late, birth rates were low, and it was difficult to replace losses from the class. To exacerbate this problem, it was possible to be demoted from Spartiate status for a number of reasons; cowardice in battle was a common one, as was inability to pay for membership in the common mess. Inability to pay became an increasingly severe problem as commercial activity began to develop in Sparta, since some Spartiates would sell the land from which they were supposed to draw their earnings. Since the constitution included no provisions for promotion to Spartiate status, the number of Spartiates gradually dwindled as the classical period wore on.
By the mid 4th century BC, the number of Spartiates had been critically reduced, although Sparta continued to hold sway over much of Greece. Finally, at Leuctra in 371 BC, a Theban army decisively defeated a Spartan force, killing 400 Spartiates and breaking the back of Spartan military power. In 370 BC, Messenia was liberated by a Theban army, destroying the basis of the Spartan social system. The Spartan state never recovered its former power, and the Spartan army, by the later 3rd century, was not particularly superior to other hoplite armies in Greece.
Not to take anything away from Epominondas or Pelopidas of course, Leuctra is still an excellent achievement.