to power an ostensible government of national unity led by the head of the Royal Yugoslav Air Force, General Dušan Simović. The coup enraged Hitler, who wished to irrevocably dismantle Yugoslavia, which he dubbed a "Versailles construct". He immediately ordered the country's invasion, which commenced on 6 April. Creation of the NDH Yugoslavia was quickly overwhelmed by the combined strength of the Axis powers and surrendered in less than two weeks. The government and royal family went into exile, and the country was occupied and dismembered by its neighbours. Serbia was reduced to its pre-Balkan War borders and directly occupied by Germany. Serb-inhabited territories west of the Drina River were incorporated into the Axis puppet state known as the Independent State of Croatia (; NDH), which included most of modern-day Croatia, all of modern-day Bosnia and Herzegovina, and parts of modern-day Serbia. The establishment of the NDH was announced over the radio by Slavko Kvaternik, a former Austro-Hungarian Army officer who had been in contact with Croatian nationalists abroad, on 10 April. Pavelić entered the NDH on 13 April and reached Zagreb two days later. The same day, Germany and Italy extended diplomatic recognition to the NDH. Pavelić assumed control and bestowed himself the title Poglavnik ("leader"). At the time of its establishment, the NDH had a population of 6.5 million inhabitants, about half of whom were Croats. It was also inhabited by nearly two million Serbs, who constituted about one-third of its total population. Nevertheless, Serbs—along with others whom the Ustaše deemed "undesirable", such as Jews and Roma—were denied citizenship on the basis that they were not Aryans, and immediate measures were taken to expunge the presence of the Cyrillic alphabet from the public sphere. On 17 April, the Ustaše instituted the Legal Provision for the Defence of the People and State, a law legitimizing the establishment of concentration camps and the mass shooting of hostages in the NDH. Thirty concentration camps in total were established across the puppet state. History Sisak I The town of Sisak, near the confluence of the Sava and Kupa rivers, is located more than southeast of Zagreb. During the war, the town hosted two sub-camps, which were initially jointly administered by the NDH authorities and the German Commissioner in Croatia (). The first sub-camp, Sisak I, served as a transit camp for thousands of captured Serbs, Bosniaks, and Roma who were to be