During the First World War Galicia saw heavy fighting between the forces of Russia and the Central Powers. The Russian forces overran most of the region in 1914 after defeating the Austro-Hungarian army in a chaotic frontier battle in the opening months of the war. They were in turn pushed out in the spring and summer of 1915 by a combined German and Austro-Hungarian offensive.
In 1918, Western Galicia became a part of the restored Republic of Poland, which absorbed the Lemko-Rusyn Republic. The local Ukrainian population briefly declared the independence of Eastern Galicia as the "West Ukrainian People's Republic". During the Polish-Soviet War the Soviets tried to establish the puppet-state of the Galician SSR in East Galicia, the government of which after couple of months was liquidated.
The fate of Galicia was settled by the Peace of Riga on March 18, 1921, attributing Galicia to the Second Polish Republic. Although never accepted as legitimate by some Ukrainians, it was internationally recognized on May 15, 1923.[5]
The Ukrainians of the former eastern Galicia and the neighbouring province of Volhynia, made up about 12% of the Second Polish Republic population, and were its largest minority. As Polish government policies were unfriendly towards minorities, tensions between the Polish government and the Ukrainian population grew, eventually giving the rise to the militant underground Organization of Ukrainian Nationalists.