The defeat of the Ukrainians' struggle for their state in 1917-1921 pushed some of them, especially the youth, towards radical nationalism. One of the main organisations founded at the time was the OUN – the Organisation of Ukrainian Nationalists, one of whose leaders was Stepan Bandera. The organisation carried out terrorist attacks in the Second Republic. The Minister of Home Affairs, Bronislaw Pieracki, died at its hands. The assassination was organised by Bandera, who was sentenced to death by a Polish court – later reduced to life imprisonment. Ukrainian nationalists were also planning the assassination of Jozef Pilsudski.
The ideologue of this organisation, Mykhailo Kolodzinski, was the first, back in the 1930s, to draw up plans for ethnic cleansing in Volhynia and Galicia.
Bandera was released from prison, abandoned by Polish guards, in September 1939. He began to collaborate with the Third Reich. On 30 June 1941, in German-occupied Lviv, the OUN declared Ukrainian independence. The Germans strongly objected and interned Stepan Bandera in response.