The former was used to intern adults destined for forced labour in the Reich and was established in 1941, while the latter was used to detain unaccompanied Serb—and to a lesser extent, Jewish and Roma—children who had been separated from their parents over the course of the conflict.

Thousands of children were saved from the camp as a result of rescue efforts spearheaded by the humanitarian Diana Budisavljević and the local communist underground.

The exact number of children who perished there is unknown, but estimates range from 1,160 to 1,600, largely as a result of starvation, thirst, typhus and neglect.

[1] Tensions flared in 1928, following the shooting of five Croatian parliamentary deputies by the Montenegrin Serb politician Puniša Račić in the country's parliament.

The Ustaše were outlawed in Yugoslavia, but received covert assistance from Benito Mussolini's Italy, which had territorial pretensions in Istria and Dalmatia.

The Ustaše carried out a number of actions aimed at undermining Yugoslavia, most notably the Velebit uprising in 1932 and the assassination of King Alexander in Marseilles in 1934.

Following Alexander's assassination, the Ustaše movement's seniormost leaders, including Pavelić, were tried in absentia in both France and Yugoslavia and sentenced to death.

[5] Intending to secure his southern flank for the impending attack on the Soviet Union, German dictator Adolf Hitler began placing heavy pressure on Yugoslavia to join the Axis.

Two days later, a group of pro-Western, Serbian nationalist Royal Yugoslav Air Force officers deposed the country's regent, Prince Paul, in a bloodless coup d'état.

[11][a] 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.

[15] 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.

[16] 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.

The first sub-camp, Sisak I, served as a transit camp for thousands of captured Serbs, Bosniaks, and Roma who were to be deported to perform forced labour in the Reich.

Euphemistically referred to as a "transit camp for refugees" by its administrators, it was established on part of an abandoned factory, which was surrounded by barbed wire.

The German authorities sent some of the able-bodied prisoners from Sisak I to the Sajmište concentration camp, directly across the border from German-occupied Belgrade.

[20] According to the historian Joseph Robert White, the first 1,200 children arrived from the Mlaka sub-camp on 29 July 1942, with subsequent transfers from Jasenovac V (Stara Gradiška) and Jastrebarsko taking place in August.

[21] According to the historians Paul R. Bartrop and Eve E. Grimm, Sisak II was officially established on 3 August 1942, following the Kozara Offensive (German: Operation West-Bosnien) against the Partisans in northwestern Bosnia.

The intelligence agency of the Nazi Party and the Schutzstaffel (SS), the Sicherheitsdienst (SD), also sent a representative to Sisak, and German field gendarmes provided security around the two sub-camps and the adjacent railway.

The poor living conditions at Sisak II shocked many observers, including Dumbović, as well as representatives of the Croatian Red Cross.

Dumbović documented the conditions at Sisak with his camera, taking 755 photographs of the emaciated children, some dead or dying, and others lying naked on the floor.

Records kept by Budisavljević containing information about each child detained at Sisak were confiscated by the Department for People's Protection (Serbian: Odeljenje za zaštitu naroda; OZNA), a Yugoslav state security agency founded in 1944.

She survived and later became the subject of an iconic war photograph titled Kozarčanka, which was widely seen as a symbol of the Partisan resistance in post-war Yugoslavia.

"Such a concept was intentional," according to the academic Sanja Horvatinčić, "and was meant to console and give hope to the survivors of the war and to the visitors who are faced with the brutal history of the site."

[26] Monuments commemorating the children who died, such as the ones at the Reis Saltworks and the Sisak Cultural Center, were destroyed in the early 1990s, during the Croatian War of Independence.

[26] In post-independence Croatia, the Sisak camp's main building was transformed into a movie theatre and renamed the Crystal Cube of Cheerfulness (Croatian: Kristalna kocka vedrine).

[35] In response, the Roman Catholic Archdiocese of Zagreb sent a letter of protest to Patriarch Porfirije, stating that "with regard to this matter, the Holy Synod of Bishops of the Serbian Orthodox Church has obviously accepted rhetoric and communist propaganda, full of untruths and manipulations, with which it is being attempted to blame innocent people for the alleged torture and murder of children, thousands of whom, owing to the love and care of Croatian Catholics, were saved from death and survived the difficult wartime conditions.