In response to the German occupation, Poles organized one of the largest underground movements in Europe with more than 300 widely supported political and military groups and subgroups. Despite military defeat, the Polish government itself never surrendered. In 1940 a Polish government-in-exile became based in London. Resistance groups inside Poland set up underground courts for trying collaborators and others and clandestine schools in response to the Germans' closing of many educational institutions. The universities of Warsaw, Cracow, and Lvov all operated clandestinely. Officers of the regular Polish army headed an underground armed force, the "Home Army" (Armia Krajowa—AK). After preliminary organizational activities, including the training of fighters and hoarding of weapons, the AK activated partisan units in many parts of Poland in 1943. A Communist underground, the "People's Guard" (Gwardia Ludowa), also formed in 1942, but its military strength and influence were comparatively weak.
With the approach of the Soviet army imminent, the AK launched an uprising in Warsaw against the German army on August 1, 1944. After 63 days of bitter fighting, the Germans quashed the insurrection. The Soviet army provided little assistance to the Poles. Nearly 250,000 Poles, most of them civilians, lost their lives. The Germans deported hundreds of thousands of men, women, and children to concentration camps. Many others were transported to the Reich for forced labor. Acting on Hitler's orders, German forces reduced the city to rubble, greatly extending the destruction begun during their suppression of the earlier armed uprising by Jewish fighters resisting deportation from the Warsaw ghetto in April 1943.
The Nazi terror was, in scholar Norman Davies's words, "much fiercer and more protracted in Poland than anywhere in Europe." Reliable statistics for the total number of Poles who died as a result of Nazi German policies do not exist. Many others were victims of the 1939-1941 Soviet occupation of eastern Poland and of deportations to Central Asia and Siberia. Records are incomplete, and the Soviet control of Poland for 50 years after the war impeded independent scholarship.
The changing borders and ethnic composition of Poland as well as vast population movements during and after the war also complicated the task of calculating losses.
In the past, many estimates of losses were based on a Polish report of 1947 requesting reparations from the Germans; this often cited document tallied population losses of 6 million for all Polish "nationals" (Poles, Jews, and other minorities). Subtracting 3 million Polish Jewish victims, the report claimed 3 million non-Jewish victims of the Nazi terror, including civilian and military casualties of war.
Documentation remains fragmentary, but today scholars of independent Poland believe that 1.8 to 1.9 million Polish civilians (non-Jews) were victims of German Occupation policies and the war. This approximate total includes Poles killed in executions or who died in prisons, forced labor, and concentration camps. It also includes an estimated 225,000 civilian victims of the 1944 Warsaw uprising, more than 50,000 civilians who died during the 1939 invasion and siege of Warsaw, and a relatively small but unknown number of civilians killed during the Allies' military campaign of 1944—45 to liberate Poland.