Sparta, their entire civilization basically atrophied because of it.

Sparta was built on slavery, their obsession with war was necessary to keep their slaves in check and deal with constant uprisings, it was very practical for their way of life.

No, it was because of the low fertility rate. Most powerful class in Sparta were rich widows, even kings had to borrow money from them.

Its also because they built it entirely on slaves and refused citizenship to most. 300 was an entertaining movie but it left out the slavery that was central to Sparta.