(sometimes called the Sacred Band
While several modern countries debate about letting gays into the military, things were quite different in Ancient Greece.
The idea of forming an entirely military unit based on fighters-lovers was conceived in Plato's Symposium. However it was not Athens, but rather Thebes that first adopted the idea and formed an elite unite and named it "The Holy Legion". The legion was based on pairs of lovers (most of them young aristocrats) who were trained to fight together, out of the assumption that no man would want to seem incompetent in battle in front of his lover, and would want to defend his lover to the best of his ability.
The Holy Legion was crucially instrumental in the Theban victory over Sparta in The Battle of Leuctra (371 BC), which ended the supremecy of Sparta in Greece that was attained after the end of the Second Peloponesian War. In all its history it never lost a single battle until The Battle of Chaironeia (337 BC) against Philip of Macedonia, in which the legionmen fought until not one of them remained alive. This battle also marked the end of the independant Theban polis.
Plutarch tells great heroic stories about the Holy Legion, the most famous of which is: a legioner and a macedonian soldier were dueling in Chaironeia, when the legioner accidentally stepped on a stone and fell down on his stomach, the macedonian was going to kill the theban, when the theban said: "Wait just a second and let me turn so that I'm on my back and then you could kill me." The macedonian wondered and asked him why he was making such an odd request, to which the legioner answered:"So that if my lover comes looking for me after the battle is over he won't find me with a sword in my back and think that I was killed while fleeing." The macedonian then let the legioner turn over and then killed him.