Leonis is one of the Twelve Colonies of Kobol. In the service, rookies are taught the "Lesson of Leonis", which is two-fold. Leonis teaches that over-enthusiastic pride can destroy. It's also a testament to the power of the colonial arsenal, and what happens when war goes too far. These lessons are quite apparent in the aftermath of the Cylon attack. There's just not many left to learn them anymore.
When the tribes originally settled the Colonies, the followers of Leo fought tooth and nail to claim one of the three habitable moons of the gas giant Zeus. The colonists knew that the planet contained uranium, which was required to make nuclear weapons. They were sure that controlling a primary source of uranium would give them great standing in the colonies. After a hard fight, they won their claim. Early on, their position was very strong. The Leonids were instrumental in founding the colonial military and advancing its weaponry. They built elaborate stations, hoping to become the hub for Fleet operations in the system.
Unfortunately, the uranium ran out within a few centuries. The Leonids continued to fight for their place at the head of the proverbial table but it only got them into trouble with the other colonies. During a particularly bloody civil war, Caprica and the other two moons of Zeus - Picon and Scorpia - bombed Leonis back to the Stone Age.
Leonis was largely subservient to Picon after that. Politically, it was a non-entity. The Quorum of the Twelve was often a Quorum of Eleven with one representative standing in for both colonies. The Leonids became bitter, resentful people. They scraped by working a number of ship-building contracts at gloomy, hollow stations. Some claimed the military continued to grant these contracts out of guilt.
Fact is, things are actually looking up for Leonids nowadays. They're making a comeback in the post-invasion fleet. Many of their people were ground pounders, deck hands, and mercenaries before the attack. Exactly the skills needed now. More and more Leonids are being recognized as heroes, leading the defense against the Cylons.