The Thebans at Thermopylae - 480 B.C.
Herodotus states that on the last day only the Spartans, Thespians and Thebans remained, the Thebans very much against their will, and were kept as hostages by Leonidas.
This is a very interesting point to have been written because the obvious first question is "How do you keep the hoplite force of the Thebans as hostages when you are in a full scale battle for your life, with little or no reserves?" And secondly, "why not just kill them?" The Spartans had killed too many helots in 'cold blood' to rule out any moralistic virture here.
The answer is easy, you can't! This is because they did not stay as hostages, but of their own free will. How can we be certain of this?
Alot of it, we admit is reading between the lines of historic text.
Herodotus wrote the book "Histories", somewhere in the beginning of the Peloponnesian War, around 430 to 420 B.C., we know this because he refers to the great war between Athens and Sparta in the book, so it was still being written in the first episodes of the Peloponnesian War. His pro-Athenian stance is clearly seen while reading the book, and he constantly refers to his Athenian sources.
During that period of time, Athens was at war with Thebes as well, who were currently getting Spartan aid and were trying to wrestle control over Boeotia away from Athens, threatening Athenian supremesy in that area. The Athenian sources would not want to give Thebes any type of valiant action in the book, and would like to portray them as cowards and allies of Persia. It was they who downplayed Thebes' role at Thermopylae and gave Herodotus information to that effect.
When Leonidas went to Thebes on his way to Themopylae, it was currently under two factions that nearly caused a civil war. One faction was pro-Persian the other pro-Hellas. Herodotus says half the Theban army joined Leonidas on his way. There is no reason to suspect that the pro-Persian side would join Leonidas. If anything they risked loosing their lives in the chaos of battle. The pro-Persian sides best bet was to sit tight and wait for the Persians to arrive (similar to what the city of Argos had also planned to do). The army that joined Leonidas was the pro-Hellas side. That would mean that when the pass at Thermopylae was lost they had no chance of returning to their city. The pro-Persian faction would have them killed and even if they didn't the advancing Persians would not have spared them.
In essence, when the other allies left to return to their cities to regroup, many of them back over the Isthmus and into the Peloponnese. Contrary to those cities, the Thebans had nothing left to return to. They were outcasts from Thebes and would not be welcomed back and with the Persians finally overcoming Thermopylae, the city of Thebes would be the first one to welcome the new visitors.
In the following year, 479 B.C. the Persians who stayed in Greece had Thebes as their main city-state base to work out of to attack Athens, the Thebans as well also fought on the side of the Persians against the Greeks at the Battle of Plataea. The pro-Hellas facton had well and truely gone by then, most (if not all) of them, dead at Thermopylae on the last day.