In April 1941, as Axis forces invaded Yugoslavia, Operation 25 initiated the establishment of the Independent State of Croatia, leading to the appointment of the Ustaše regime. The Ustaše, a nationalist and fascist organization led by Ante Pavelić, quickly took control with the backing of Nazi Germany and Fascist Italy. This new state was proclaimed a puppet regime of the Axis powers, allowing for the implementation of extreme nationalist policies, including the widespread persecution of Serbs, Jews, Roma, and political dissidents who were opposed to their rule.

Ante Pavelić, returning from exile, became the leader of the Ustaše and began implementing policies that aimed to create a homogenous Croatian state. These included forced conversions, mass killings, and the establishment of concentration camps, which were sanctioned and supported by the Nazis as part of their broader war goals. The Ustaše's collaboration with the Axis powers allowed for a radicalization of political ideology and violence that would become a hallmark of their administration.

The rapid establishment of the Ustaše government resulted in the immediate upheaval of social order, economic conditions, and communal relations in the region. Within weeks, opposition was met with severe reprisals, and citizens lived under a regime that encouraged ethnic cleansing and terror. This dark era marked a significant shift in Croatian history and set a precedent for brutality that impacted the region long after World War II concluded.