The elite Sacred Band of Thebes, an ancient Greek military unit made up of 150 same-sex couples. Formed in 378 BCE, the unit fought many victorious battles until their defeat in the Battle of Chaeronea in 338 BCE. Their structure was unique, with the theory being that same-sex couples, driven by love and a desire to protect their partner, would fight harder on the battlefield. The main account of the Sacred Band comes from Plutarch’s Lives, based on works by Callisthenes and Ephorus, both lost to history.
  

Key Points: 

1. The Sacred Band of Thebes was formed by Theban General Gorgidas in 378 BCE, composed of 150 same-sex couples chosen for their physical prowess and fighting qualities. They trained rigorously in wrestling, horsemanship, and dancing, which led to a highly bonded, elite unit.
2. The concept was rooted in an ancient Greek idea that same-sex couples, driven by their love and a commitment to protect their partner, would be stronger and more effective fighters than single heterosexual soldiers. This theory proved true in practice, as the Sacred Band won many victories in significant conflicts.
3. The Band’s strength and success were tested and confirmed in several key battles, including the Battle of Tegyra and the Battle of Leuctra. These victories significantly challenged the established military dominance of Sparta.
4. The ultimate defeat of the Sacred Band came in the Battle of Chaeronea in 338 BCE, against the forces of Macedonia. This defeat effectively ended the dominance of Thebes and marked the rise of Macedonia.
5. The story of the Sacred Band is primarily documented in Plutarch’s Lives, though it’s believed that this account was largely based on works by contemporaries Callisthenes and Ephorus, whose original works have been lost to history.

Source: https://www.worldatlas.com/ancient-world/the-elite-special-forces-of-ancient-greece.html