I had that with the Koinon. In Sparta you get a unit of Spartiates Hoplitai. I fought many battles against the Macedons and Epirotes with them, they were almost always the first to enter a city, the ones who could hold the line the longest, etc... I didn't dream of losing them. They only suffered minimal losses.

But then in a battle against some descendant of Pyrrhus and an allied army of his, I lost track of them, they were chasing some slingers uphill and I failed to notice they came to close to the Epirotes' second army. Their general charged them (he had more men than my poor spartans). Their numbers started dropping fast. I didn't want to lose them, so I send two units of Thessalian cavalery almost a quarter of the map away to them to help, the heavy cavalery was almost exhausted when reaching them, but luckily so was the general. They managed to slauger the Epirote's second army and save 10 Spartans. No Thermopylae!
NOw I've send those 10 home to act as bodyguards for the king.