3 month war between Tieflings and Humans.
As humanity was expanding through their Obelisk, they started to settle new colonies and explore dormant Obelisks.
Unbeknownst to them, they settled into the Obelisk 314. Now to humans, this was another Obelisk. But to the Tieflings who are part of the Council, this was restricted space. But instead of negotiating, the Tieflings opened fire. One starship escaped to warn the Alliance; a retaliatory force destroyed the Tiefling vessels, and the situation quickly escalated into war.
To the Tieflings, it was merely a police action against an ignorant species violating Council law; to humanity, it was their first encounter with another intelligent race - and it would shape their views of the world for decades to follow.
Fortunately, the Council then intervened to negotiate a peace and bring the war to an end. From the Council, humans learned of the existence of the Citadel, and that there were many more sentient species in the galaxy who co-existed peacefully. In the end, only six hundred and twenty-three human lives were lost with slightly more tiefling casualties.