During the Polish Defensive War of September to October 1939 the garrison of Grodno was mostly used for the formation of numerous military units fighting against the invading Wehrmacht. In the course of the Soviet invasion of Poland (initiated on 17 September 1939) heavy fighting took place in the city between Soviet and improvised Polish forces, composed mostly of march battalions and volunteers. In the course of the Battle of Grodno (20-22 September) the Red Army lost some hundred men (according to Polish sources; according to Soviet sources – 57 killed and 159 wounded) and also 19 tanks and 4 APCs destroyed or damaged. The Polish side suffered at least 100 killed in action, military and civil, but losses still remain uncertain in detail (Soviet sources claim 644 killed and 1543 captives with many guns and machine guns etc. captured). Many more Poles were shot in mass executions after being imprisoned. After the Soviet forces surrounded the engaged Polish units, the escaping Polish units withdrew to Lithuania.