Resistance continued in many places. Warsaw was eventually bombed into submission. The event that served as a trigger for its surrender on 27 September was the bombing damage to the water supply system caused by deliberate targeting of the waterworks. Warsaw suffered the greatest damage and civilian losses (40,000 killed), already in September 1939. The Modlin Fortress capitulated on 29 September, the Battle of Hel continued until 2 October, and the Battle of Kock was fought until 4 October. In the country's woodlands, army units began underground resistance almost at once. Major "Hubal" and his regiment pioneered this movement. During the September Campaign, the Polish Army lost about 66,000 troops on the German front; about 400,000 became prisoners of Germany and about 230,000 of the Soviet Union. 80,000 managed to leave the country. 16,600 German soldiers were killed and 3,400 were missing. 1000 German tanks or armored vehicles and 600 planes were destroyed. The Soviet Army lost between 2,500 and 3,000 soldiers, while 6,000 to 7,000 Polish defenders were killed in the east. Over 12,000 Polish citizens executed by the Nazis were among the approximate 100,000 civilian victims of the campaign.