The Thirdspace Invasion occurred after the Second Shadow War and before Sheridan's Campaign to liberate Earth. It began with the discovery of a Vorlon-constructed artifact that led into another universe called Thirdspace. This alternate dimension was inhabited by a race of hostile, very powerful telepathic aliens that had attempted to invade our universe thousands of years before. Though the Vorlons had repelled the invaders, the aliens enslaved several Vorlons and used them to fling the portal into hyperspace. It was rediscovered in June 2261 by a team of humans, who then brought it to Babylon 5 study. The Thirdspace aliens became aware of this and planned again to invade our universe and destroy all life.
Using the artifact as a conduit, the aliens telepathically enslaved some of the research team, and used them to fully reactivate the portal. The aliens also took control of much of the station's civilian population and turned them against Babylon 5's security personnel. At the same time, dozens of alien fighters emerged from the portal. Starfurys, White Stars, and Minbari warcruisers all converged on the portal to intercept the invaders, but the alien fighters proved to be formidable and the allied fleet was hard-pressed to even keep them at bay.
Throughout all this, Captain John Sheridan, geared up in a space suit, entered the portal and crossed into Thirdspace, bringing a thermonuclear bomb with him. He planted the bomb and escaped, just as the Thirdspace invaders began sending their larger capital ships into the normal universe. The bomb detonated and destroyed the portal, along with at least one capital ship. The allied fleet then destroyed the remaining alien fighters.
With the Thirdspace portal destroyed, the civilians on Babylon 5 were freed from the aliens' influence. 
|Wars and Conflicts|
|Last Great War • First Shadow War • Second Shadow War|
|Chilean War • World War III • War of the Shining Star • Dilgar War • Canal Wars • Earth-Minbari War • Earth Alliance Civil War • Telepath War • Second Earth Alliance Civil War|
|War of 20 Million Deaths • 1st Centauri Occupation of Narn • Narn-Centauri War • 2nd Centauri Occupation of Narn • Centauri Wars of Aggression • Centauri War of 2262|
|League Wars of 2260 • Minbari Civil War • Drakh War • The Great Trade War|
|Battle of the Scoria Plains • Battle of Nu'Shok • Battle of Douala • Liberation of the African Block • Battle of Janos VII • Invasion of Ragesh III • Raider Attack on Babylon 5 • Battle for Epsilon III • Operation Sudden Death • Thirdspace Invasion • "Scout Fleet" Assault on Babylon 5|