Mass Effect, Mass Effect 2 & Mass Effect 3
In the early 2160s, the Alliance began aggressive colonization of worlds in the Skyllian Verge, much to the dismay of batarians who had been developing the region for several decades. In 2171, the batarians petitioned the Council to declare the Verge a "zone of batarian interest." The Council refused, however, declaring unsettled worlds in the region open to human colonization.
In protest, the batarians closed their Citadel embassy and several official diplomatic relations with the Council, effectively becoming a rogue state. They instigated a proxy war in the Verge by funneling money and weapons to criminal organizations, urging them to strike at human colonies.
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.