Before the war Poland as doomed to become a failed state and soon collapse anyways, their Anti-Ukrainian policies which banned practically the Language, Schools and even Churches and this happened before, you would think that such policies come without reprisal. http://en.wikipedia.org/wiki/History_of_the_Ukrainian_minority_in_Poland#Education
Ukrainian language usage was negatively impacted by the system. A law setting up bilingual Polish-Ukrainian schools and Polish schools, passed in 1924 by the government of Władysław Grabski, resulted in a rapid decline in the number of uni-lingual Ukrainian schools (from 2,426 in 1922 to 352 in 1938 in Galicia; and from 443 in 1922 to 8 in 1938 in Volhynia) and increase in Polish-Ukrainian bilingual schools (2,485 in Galicia; 520 in Volhynia) and Polish schools.[70] By the 1930s these "bilingual" schools became effectively Polish.[71]
Eventually, a hundred ninety Orthodox churches were destroyed and often abandoned[61] and another one hundred fifty were transformed into Roman Catholic churches. As a result, out of 389 Orthodox churches in Volhynia in 1914, only 51 remained in 1939. In addition to losing church buildings, the Orthodox Church lost large areas of land, which were taken the Polish state and kept by it, or given to the Roman Catholic Church.[54] In the regions of Chelm and Polisia, armed groups of Polish colonists known as Krakus terrorized Ukrainian civilians into converting to Catholicism.[62] Remaining Orthodox churches were forced to use the Polish language in their sermons. The last official government act of the Polish state in Volhynia was to, in August 1939, convert the last remaining Orthodox church in the Volhynian capital of Lutsk into a Roman Catholic one.[32] The Orthodox clergy in Volhynia used the persecution of their church to build up already strong feelings of resentment among the local Ukrainian people against the Poles.[54]
When Poland tried to silence and remove a minority they should not be surprised when they later take up arms against their oppressors.