'Ostra Brama' was the Polish uprising by the Armia Krajowa (Home Army) in Vilnyus, then technically still a part of Poland, as an element of the 'Burza' Polish national rising against the Germans (7/14 July 1944).
Though the Germans were defeated, on the following day Soviet forces entered the city and the NKVD (secret police) then interned Polish soldiers and arrested their officers. Several days later, the remains of the Armia Krajowa retreated into the nearby forests, leaving the Soviets were in control of the city.
There are some controversies involved in determining the result of the battle: from the Polish point of view, while the German defeat constituted a Polish tactical victory, the ensuing destruction of the Polish units by the Soviets resulted in a strategic defeat, especially in light of the goals of 'Burza', and from the Soviet point of view, the operation was a complete success, as both the Germans and the Poles loyal to the Polish government-in-exile in London suffered a defeat.
On 12 June 1944 Generał dywizji Tadeusz Bór-Komorowski, commander of the Armia Krajowa, ordered the development of a plan to free Vilnyus from German control, and the Home Army districts of Vilnyus and Navahrudak planned to achieve this before the Soviets could reach Vilnyus. The commander of the Armia Krajowa district in Vilnius (Wilno in Polish), Generał brygady Aleksander Krzyżanowski, decided to regroup all the guerrilla units in north-east Poland for the assault, from both inside the city and outside.
The start date for 'Ostra Brama' was fixed as 7 July, and some 12,500 Armia Krajowa soldiers attacked the German garrison and managed to seize most of the city centre, although heavy fighting continued in the city’s outskirts to 14 July. In the city’s eastern suburbs, the Armia Krajowa units co-operated with reconnaissance groups of General Ivan D. Chernyakhovsky’s 3rd Belorussian Front, approaching the city as part of its 'Vilnyus Offensive Operation'.
Krzyżanowski wished to group all of the partisan units into a re-created Polish 19th Division. However, as the Soviet forces advanced into the city on 15 July, the NKVD secret police started to intern all Polish soldiers. On 16 July the headquarters of the 3rd Belorussian Front invited Polish officers to a meeting and there arrested them. The internees, almost 5,000 officers, non-commissioned officers and soldiers, were sent to a provisional internment camp in Medininkai, a suburb of Vilnyus. Some of them were given the option of joining the Polish 1st Army, which was integrated into the Soviet armed forces, while the majority were sent to prisons and GULAGs in the USSR.
Subsequently, the remnants of the local Armia Krajowa headquarters ordered all surviving elements to retreat to the Rudininkai forest. It is estimated that by 18 July almost 6,000 soldiers and 12,000 volunteers had reached the area. They were soon discovered by Soviet air reconnaissance and surrounded by the NKVD. The commanders decided to split their units and try to break through to the Białystok area, but most of the Armia Krajowa units were caught and interned. An unknown number of soldiers under Podpułkownik Maciej Kalenkiewicz remained in the forests around the city until a time early in August. On 21 August a minor engagement was fought between these Armia Krajowa elements and the NKVD, and the fate of the Poles has never been established.