to 30 September 1929 the fairs site became the venue for a major National Exhibition (Powszechna Wystawa Krajowa, popularly PeWuKa), held to mark ten years of Polish independence. The exhibition attracted around 4.5 million visitors. The Pan-Slavic Congress of Singers was held as part of the exhibition's opening days. In the interwar period the city's borders were expanded to include Główna, Komandoria, Rataje, Starołęka, Dębiec, Szeląg and Winogrady (including the Citadel) in 1925, and Golęcin and Podolany in 1933. The city's area was now . World War II Poznań was invaded by German troops on 10 September 1939, during the invasion of Poland, which started World War II. On 12 September, the Einsatzkommando 1 and Einsatzgruppe VI paramilitary death squads entered the city to commit various crimes against the population. Already in September, the occupiers carried out mass arrests and first massacres of local Poles, including at nearby Zdziechowa. The Germans also carried out mass searches of Polish institutions and libraries, and closed down Polish organizations, press and cultural institutions. Poznań was made the headquarters of the central district of the Selbstschutz, which task was to commit atrocities against Poles during the German invasion of Poland. On 8 October 1939, Poznań was annexed by Germany, and was included into a newly formed province called initially Reichsgau Posen, and later Reichsgau Wartheland (Warthe being the German name for the Warta river). The governor (Gauleiter) was Arthur Greiser, who would be hanged for crimes against humanity after the war. Ernst Damzog was appointed the police inspector for both Sicherheitspolizei and Sicherheitsdienst in German-occupied Poznań. During the German occupation, many people were murdered, executed, tortured or detained in inhumane conditions, particularly at the notorious Fort VII concentration camp, set up in one of the late 19th-century defensive forts, and later at the camp in Żabikowo. In October 1939, the German police and Selbstschutz also carried out mass arrests of local priests (see Nazi persecution of the Catholic Church in Poland). Over 3,400 Polish prisoners of Fort VII, including college students, women and nuns, were massacred in nearby Dopiewiec and Dębienko. Several local Polish school principals and teachers were also murdered in the Mauthausen concentration camp. Mentally ill patients from the local municipal hospital were gassed in Fort VII, as part of Aktion T4. Some 100,000 inhabitants were expelled to the General Government in the more-eastern part of German-occupied Poland. The aim of German