Ukrainians were also another people with strong anti-Polish hostility. The Polish–Ukrainian War of 1919 resulted in Ukraine crippled militarily and, though Poland did assist Ukraine in the eventual conflict against the Bolsheviks, but was unable to prevent an eventual occupation by the Soviets. This had led to enmity against Poland by Ukrainian nationalists, which resulted in the establishment of Organization of Ukrainian Nationalists and the beginning of Ukrainian problem in Poland. Assassinations of Polish officials by Ukrainian nationalists became increasingly frequent from 1930s onward.