On July 26, the Supreme Council of the Entente assigned Sejny and Suwałki districts to Poland with the Foch Line.

[3] The Suwałki district of the Polish Military Organisation (POW) (founded in early 1919) prepared the Sejny uprising, which took over the city from the Lithuanian authorities on August 23.

After the Bolsheviks captured Vilnius and Grodno on July 14 and 23, respectively, Polish troops withdrew to the Suwałki region.

On July 12, 1920, the Soviet–Lithuanian Peace Treaty ending the Lithuanian–Soviet War was signed after months of negotiations, whereby areas in the Suwałki region (also known as South Užnemunė in Lithuanian) were recognized as part of Lithuania.

[3] While the Polish troops retreated due to Soviet attacks, the Lithuanian army's Marijampolė Group started its march on July 19.

[3] Following orders of Edward Rydz-Śmigły, Polish forces took Augustów from Lithuanians in a surprise attack on August 28.

[3] Attacking on both sides of the Kalvarija–Suwałki road on September 4, the right column reached the Gulbieniszki–Czerwonka–Kaletnik line, fighting with the Polish near Żubryn and Gulbieniszki, but failing to capture Suwałki.

[8] On the Augustów-Lipsk section, the Polish army units, reinforced by a cavalry regiment and an infantry battalion, took over Jastrzębna, so the left column, avoiding encirclement, withdrew from Augustów.

[3] Sections to be defended by the Lithuanian army were distributed by the command regardless of the area and available forces, without creating any reserves.

[3][9] The 3rd Infantry Division led by Ignas Musteikis that was moved on September 9 to the Suwałki Region to stabilize the Lithuanian army's situation was deployed from the Vištytis Lake to Żegary.

[3] On 8 September, during a planning meeting of the Battle of the Niemen River, the Poles decided to maneuver through the Lithuanian-held territory to the rear of the Soviet Army, stationed in Grodno.

[2] Polish Northern Assault Group, consisting of infantry and cavalry divisions, was ready to enter action on 19 September, with the railroad hub of Lida set as its objective.

The most important (wing) group of the Polish 2nd Army consisted of the:[3] The Lithuanian forces had their two divisions assigned to two lines of defense, and outdated static and linear tactics were followed.

They pushed out the Lithuanian 5th and 8th Infantry Regiments from Zelwa, Budwiecie and Berżniki, began to surround Sejny from the east, while others attacked from Wiłkopedzie in the north.

[3] This defeat meant that the lands where many ethnic Lithuanians inhabited were lost to Poland and the Polish army, having moved across the Neman, occupied southeastern Lithuania, meaning convenient starting positions for gen. Lucjan Żeligowski's forces to occupy Vilnius during his false-flag mutiny in early October.