[2]: 82  It was fought between improvised Polish units under Gen. Wacław Przeździecki and Soviet Red Army troops of Komkor Ivan Boldin's Dzerzhinsky Cavalry Mechanized Group, at the time in a non-aggression agreement with Nazi Germany under the Ribbentrop-Molotov Pact.

The Soviet aggression caught much of the eastern Poland virtually undefended, as most of the Polish forces from the area had already been transferred to the German front.

From the excess of troops there was formed the Operational Group "Grodno" (about 7 thousand soldiers) with a goal of defending western and northwest sides of the city.

Up to 12 September, the certified leader of the Fortified Zone of Grodno was colonel Bohdan Hulewicz, who made orders for defense of the city, however, against German troops.

On 17 September, as a result of rumours about Soviet aggression on Poland, a communist sabotage happened to the city, which was however suppressed quickly.

He did not believe the city could or should be defended any longer and ordered some troops to retreat into the region of Sopoćkiń and Kalet to cover forces moving into Lithuania.

In the early morning of 21 September, the defenders were joined by the remnants of the reserve Wołkowysk Cavalry Brigade under Brigadier General Wacław Przeździecki.

However, Polish historians Andrzej Krzysztof Kunert and Zygmunt Walkowski claim that the Red Army lost around 800 killed, missing or wounded.

Polish losses, both civilian and military, remain unknown, although Soviet records claim 644 killed and 1,543 captured (66 officers and 1,477 soldiers).

[5] One notable casualty on Polish side was 15-year-old Tadeusz Jasiński who was captured throwing a petrol bomb and tied to a Soviet tank as a human shield.