Polonization was a policy of forced cultural and linguistic assimilation practiced by the Polish government in Western Ukraine in the interwar years. The Treaty of Versailles frustrated Ukrainian hopes for an independent state; Western Ukrainian territories, the largest of which was Galicia, were incorporated into the new Polish state. However, the Treaty of Versailles guaranteed minority rights. The Polish government, however, did nothing to protect the rights guaranteed with Ukrainians – in fact, it actively suppressed the Ukrainian population.

The policy of Polonization was carried out chiefly in three main directions – cultural, economic and political. The Ukrainian language was increasingly attacked; Ukrainian universities and schools were routinely closed. In 1931, there were no Ukrainian universities or colleges, and only four secondary schools taught in Ukrainian, as opposed to 775 Polish-language universities. The Ukrainian language was banned in all government institutions. Ukrainian-language newspapers were routinely shut down. Ukrainian organizations such as PLAST, a scouting organization, were banned.

In terms of economics, the Polish government set out to colonized the Western Ukrainian settlers with Polish settlers. In Western Ukraine, ninety percent of the peasants were Ukrainian; in 1931 they controlled only 51% of arable land, while the Polish minority, 8.5% of the peasantry, controlled 20.3%. 26.4% of arable land was redistributed between 15 000 Polish families resettled in Western Ukraine. Politically, the Polish government used pressure to ensure that ethnic Poles were disproportionately elected to all governing bodies.

Ukrainians, predictably, reacted to the policy of Polonization with strong opposition. The Organization of Ukrainian Nationalists (OUN), formed in 1929, began a policy of sabotage and led boycotts of Polish monopolies, especially tobacco and alcohol. OUN also carried out several assassinations of Polish officials.

In the 1930s, under General Pilsudski, President of Poland, the country slid increasingly into presidential authoritarianism and the legislative powers of the sejm and other state organs increasingly subordinated to the will of the President. The home rule guaranteed to Ukrainians by the Treaty of Versailles was increasingly limited. Part of this descent into authoritarianism was increased repression against Ukrainians; OUN leaders were arrested, students suspected of ties with OUN were expelled from schools, and the peasantry was routinely harassed in a program called pacification.  The policies carried out by the Polish government in the interwar years fuelled resentment between Ukrainians and Poles, which would break into open conflict a few years later, in the second half of WWII.