The Narn were visited by the Vorlons a long time back in their ancient history leaving behind genetic manipulation in the form of telepaths.
In 1260 the Shadows used part of Narn as a base for the First Shadow War and tried to cull all telepaths from the population. The Narn resisted and managed to drive the Shadows away.
In 2109 the Narn were enslaved by the Centauri and their resources exploited. Eventually resistance grew, and the Narn reverse-engineered Centauri technology to drive them away around 2210.
In 2259 the Narn declared war on the Centauri, but their technology was no match for the Centauri, and the Narn fleet were decimated.
In the Shadow War, the few remaining Narn ships joined an alliance and fought the Shadows and Vorlons.
After Narn was liberated they joined the Interstellar Alliance.