[1] After the war, in 1920–1921, approximately 100,000 Ukrainians were interned in concentration camps[6][better source needed] by the Polish government, where they were often denied food and medicine; some of them died from starvation, disease or suicide.

[7] The death toll at these camps from diseases was estimated at 20,000 people (during the war, the Ukrainian government[clarification needed] had interned 25,000 Poles).

Polish-Ukrainian relations deteriorated during the Great Depression, leading to much economic disruption, hitting hard, particularly the rural areas.

[9] In July 1930, activists of the extremist Organization of Ukrainian Nationalists (OUN) began sabotage actions, during which warehouses and cereal fields owned by Poles were burned, Polish homes were destroyed, bridges were blown up, state institutions, rail lines and telephone connections were damaged.

[1][12] The organization reacted by adopting a tactic designed to radicalize Ukrainian public opinion and block any form of compromise with Polish authorities.

[13] OUN directed its violence not only against the Poles but also against all Ukrainians wishing for a peaceful settlement of the Polish-Ukrainian conflict.

Offices of the Polish paramilitary Riflemen's Association were burned, as were the stands of the popular trade fairs in Lwów (Lviv).

The decision to carry out the action was made by Marshal Józef Piłsudski in his capacity as Prime Minister of the Second Polish Republic.

[citation needed] Before the action commenced, around 130 Ukrainian activists, including a few dozen former Sejm (Polish parliament) deputies were arrested.

Subsequently, the houses of those suspected of cooperation with Organization of Ukrainian Nationalists were searched, which included the tearing up of floors and ceilings.

[citation needed] Ukrainian nationalists lodged an official complaint regarding the "pacification" action to a committee of the League of Nations, which in its response disapproved the methods used by the Polish authorities, but also put blame on the Ukrainian extremist elements for consciously provoking this reaction from the Polish government.

[21] According to Ukrainian-Canadian historian, Orest Subtelny, "collective punishment" meted out on thousands of "mostly innocent peasants" resulted in the exacerbation of animosity between the Polish state and the Ukrainian minority.