Hostilities peaked with the Skyllian Blitz of 2176, an attack on the human capital of Elysium by batarian-funded pirates and slavers. In 2178, the Alliance retaliated with a crushing assault on the moon of Torfan, long used as a staging base by batarian-backed criminals. In the aftermath, the batarians retreated into their own systems, and are now rarely seen in Citadel space.
2008, BioWare, Mass Effect (Science Fiction), Redwood City: Electronic Arts, PC, scene: Batarians Codex entry