I see a lot of conflicting viewpoints on Sparta's military strength now. Most things I see online now are of the opinion that they were overrated and simply had very effective propaganda. A class I took with a professor who specialized in ancient Sparta seemed convinced that they in fact backed up their reputation with results all the way from the 2nd Messenian War to the start of the Theban Wars, when their citizen population was no longer large enough to support wars like they had.
A similar question was answered by /u/Iphikrates in this thread. I think you'll find what you're looking for there. He has also answered loads of other questions about the Spartans, so you might want to check his profile.