autonomy and religion. Recurring political trials were organized in the city to purge "Ukrainian nationalists", "Western spies" and opponents of Joseph Stalin inside the Bolshevik party. During this time, numerous historic churches were destroyed or vandalized and the clergy repressed. In the late 1930s, clandestine mass executions began in Kiev. Thousands of city residents (mostly intellectuals and party activists) were arrested in the night, hurriedly court-martialed, shot and buried in mass graves. The main execution sites were Babi Yar and the Bykivnia forest. Tens of thousands were sentenced to GULAG camps. In the same time, the city's economy continued to grow, following Stalin's industrialization policy. World War II During the Second World War, Nazi Germany occupied the city on 19 September 1941 (see the Battle of Kiev). Overall, the battle proved disastrous for the Soviet side but it significantly delayed the German advances. The delay also allowed the evacuation of all significant industrial enterprises from Kiev to the central and eastern parts of the Soviet Union, away from the hostilities, where they played a major role in arming the Red Army fighting the Nazis (see, for example, the Arsenal Factory). Before the evacuation, the Red Army planted more than ten thousand mines throughout the city, controlled by wireless detonators. On 24 September, when the German invaders had settled into the city, the mines were detonated, causing many of the major buildings to collapse, and setting the city ablaze for five days. More than a thousand Germans were killed. Babi Yar, a location in Kiev, became a site of one of the most infamous Nazi WWII war crimes. During two days in September 1941, at least 33,771 Jews from Kiev and its suburbs were massacred at Babi Yar by the SS Einsatzgruppen, according to their own reports. Babi Yar was a site of additional mass murders of captured Soviet citizens over the following years, including Ukrainians, Romani, POWs and anyone suspected in aiding the resistance movement, perhaps as many as 60,000 additional people. In the "Hunger Plan" prepared ahead of the Nazi invasion of the Soviet Union, with the aim of ensuring that Germans were given priority over food supplies at the expense of everyone else, the inhabitants of the city were defined as "superfluous eaters" who were to be "gotten rid of" by the cutting off of all food supplies to the city – the food to be diverted to