Thebes was one of many city states in ancient Greece. Under Spartan control for most of its ancient history, Thebes broke away during a revolt from 379 to 371 B.C. and their surprising victory over Sparta in the Battle of Leuctra under their great military general Epaminondas in 371 B.C. not only solidified their independence, but temporarily made them the most powerful city state in Greece. They were so feared that even traditional rivals Athens and Sparta formed an alliance against them. Theban supremacy ended soon however with the rise of Philip of Macedon who took prominence and of course under his son Alexander the Great.
See also
Sources
Encyclopedia of Military History, Dupuy & Dupuy, 1979