The 'Battle of Wilno' (modern Vilnius in Lithuania) was fought between Soviet and Polish forces as the former invaded Poland from the east even as the Germans continued their 'Weiss' (i) invasion of that country from the west (18/19 September 1939).

This pincer assault of Poland from west and east was in accord with the Molotov-Ribbentrop Pact of 23 August 1939. On 18/19 September, Soviet forces seized Wilno while the Polish forces, concentrated in the west against the Germans' earlier invasion, were relatively weak in the east. The Polish commanders, unsure of whether or not to oppose the Soviet entry into Poland, did not use the full defensive capabilities of the city and nearby fortifications, although the outcome of the battle would probably not have been different even if they had, given the Soviet forces' overwhelming numerical superiority.

The city of Wilno was the capital of Poland’s Wilno voivodeship and the sixth largest city of the 2^nd Polish Republic, in addition to being and an important industrial centre in the north-east of that country. Administratively, it was part of the Grodno-based III Military Corps Area and under the command of Pułkownik Józef Olszyna-Wilczyński, it was also an important garrison and the mobilisation centre for the 35^th Division. In the period between the world wars, the city had housed the entire 1^st Legions Division, as well as the headquarters and the 4^th Niemen Uhlan Regiment of the Wileńska Cavalry Brigade. Air cover was provided by the majority of the 5^th Aviation Regiment stationed at the nearby Porubanek air base.

Before the outbreak of war, the 1^st Legions Division had been secretly mobilised and sent towards Różan in northern Mazovia. The Wileńska Cavalry Brigade soon followed, and in the first days of September left the city for Piotrków Trybunalski. The air assets were attached to the Armia 'Modlin' and the Operational Group 'Narew' fighting the German formations and units breaking break into northern Poland from East Prussia. By 7 September the 35^th Division had been fully mobilised and transported to Lwów, and Wilno was this left defenceless.

The city’s military commander, Okulicz-Kozaryn, had decided that in the event of any attack by German or Soviet forces, he lacked sufficient forces for a successful defence, and thus that his task could be only to allow civilians to evacuate to neutral Lithuania: this had also been realised, albeit not very clearly, by Generał dywizji Józef Olszyna-Wilczyński, commander of the III Military Corps Area in which Wilno lay.

On 17 September, Wilno had 14,000 soldiers and militia volunteers, of whom only 6,500 were armed. Before the battle, the numbers of armed soldiers rose slightly as a number of disorganised units trickled in, while the number of unarmed volunteers decreased as Okulicz-Kozaryn ordered unarmed volunteers not to become involved in hostilities. Before the arrival of the Soviet forces, the Polish forces constituted about 10 infantry battalions, supported by some 15 light artillery and anti-tank guns and about five anti-aircraft guns. The defenders also had some 40 machine guns.

On 18 September, the commander of the Belorussian Front, Komandarm Mikhail P. Kovalyov, ordered that the city be taken by the 3^rd Army and 11^th Army. The 3^rd Army then assigned the 24^th Cavalry Division and the 22^nd and 25^th Armoured Brigades, under Kombrig Piotr N. Akhlyustin, to advance on the city from the north-east and the 11^th Army allocated the 36^th Cavalry Division and the 6^th Armoured Brigade, under Kombrig Semyon P. Zybin, to advance on the same city from the south-east. Their task was to take the city by the evening of 18 September, but then as a result of logistical difficulties and an overestimation of the Polish defences, the operation was revised with the aim of securing the city by the morning of 19 September.

At about 17.00 on 18 September, Okulicz-Kozaryn received reports of Soviet forces approaching from Oszmiana (now Ashmyany). These were armoured reconnaissance units, which had engaged Polish infantry units on their approach. Okulicz-Kozaryn then ordered all his units to fall back toward the border with neutral Lithuania, with units of the Border Protection Corps, as the most experienced available to him, screening the withdrawal. Podpułkownik Podwysocki was despatched to inform the Soviets that Polish forces did not intend to defend the city, but he was taken under Soviet fire and returned to the Polish lines. As Okulicz-Kozaryn had already left the city, Podwysocki decided to defend it, even though most of the forces previously in the city had left with Okulicz-Kozaryn.

The first Soviet attack on the evening of 18 September was repulsed by the Polish defenders, but subsequently the Soviets continued to push into Wilno. By the end of the day the Soviets had secured the airfield and made several thrusts into the city, taking the Rasos cemetery. By the morning of 19 September, the advanced Soviet armoured units had been reinforced with infantry and cavalry. The Polish defenders delayed the Soviet advance, particularly by holding the bridges, but later that day the poorly co-ordinated Polish defence collapsed and the Soviets took control of the city.

Polish units had either surrendered or withdrawn, in a disorganised fashion, toward the Lithuanian border or deeper into Poland. The Soviets transferred Wilno to Lithuania in accord with the Soviet-Lithuanian Mutual Assistance Treaty of 10 October 1939, and Lithuanian troops entered the city on 27/28 October.

The defence of Wilno has been criticised by some Polish historians, who point out that had they been organised effectively, the Polish forces would have been able to hold and delay the Soviets for several days in a manner similar to that of the defence of Grodno, in which some of the units which withdrew from Wilno took part. Nonetheless, this could have only been a symbolic defence, as the Polish forces had no realistic way to halt the overwhelming Soviet advance.