In January 1919, the Army of Soviet Latvia (ASL) emerged out of Red Army units fighting on future Latvian territory. Until its dissolution in early June 1919, units of the ASL participated in a war that covered areas claimed by both the Latvian and Lithuanian governments. The article aims to reevaluate the campaign in northern Lithuania in the first half of 1919. Until today, the fighting on the left flank of the ASL has been seen as a secondary front, and therefore usually overlooked. The article explores the plans of the ASL, the forces involved, and the actual warfare. Attention is also paid to events behind the front line, and the activities of the Soviet Latvian authorities in Lithuania.