Hostilities over the Suwałki Region resumed in summer 1920. When the Polish Army began to retreat during the course of the Polish–Soviet War, the Lithuanians moved to secure what they claimed to be their new borders, set by the Soviet–Lithuanian Peace Treaty of July 1920. The Peace Treaty granted Sejny and surrounding area to Lithuania. Poland did not recognize this bilateral treaty. Ensuing tensions heightened until the outbreak of the Polish–Lithuanian War. Sejny changed hands frequently until it was controlled by Polish forces on September 22, 1920. The situation was legalized by the Suwałki Agreement of October 7, 1920, which effectively returned the town to the Polish side of the border.