Polish–Ukrainian War
The Polish-Ukrainian War of 1918 and 1919 was a conflict between the forces of Poland and Western-Ukrainian People's Republic for the control over the Eastern Galicia after the dissolution of Austria-Hungary.
The first clash happened on November 1, 1918, in the city of Lemberg (called Lwów by the local Poles and Lviv by the Ukrainians]]. The Ukrainian militias under D. Vitovskyi took the advantage during the dissolution of Austria-Hungary and took control over most of Eastern Galicia. The Western-Ukrainian People's Republic was proclaimed with Lviv as its capital, and claimed sovereignty upon Eastern Galicia, Carpathians up to the city of Nowy Sącz in the west, Carpathian Ruthenia and northern Bukovina. However, large part of the claimed territory was also considered Polish by the local populations.
Upon entering Lviv, the Ukrainian forces were successfully opposed by a local self-defence units formed mostly of WWI veterans, students and children. After two weeks of heavy fights within the city, an armed unit under command of Lt. Colonel Michał Karaszewicz-Tokarzewski of the renascent Polish Army broke through the Ukrainian siege and arrived to the city. On November 21 the siege was broken and the Ukrainians were repelled. However, heavy fights for other cities claimed by both Poles and Ukrainians continued, and the fights for Lwów lasted until May of 1919.
Since December of 1918 also fights started in Volhynia. Polish units were trying to seize control over the region, while at the same time the forces of the Ukrainian People's Republic under Semen Petlura tried to expand their territory westwards, towards the city of Chełm. After two months of heavy fights the conflict was resolved in March of 1919 by fresh and well-equipped units under General Edward Rydz.
On May 14, 1919 the Polish general offensive in Volhynia and Eastern Galicia started. It was carried out by units of the Polish Army aided by the newly-arrived forces of General Józef Haller de Hallenburg. The Ukrainian lines were broken, mostly due to withdrawal of the elite Sich Rifles. On May 27 the Polish forces reached the Złota Lipa-Brzeżany-Jezierna-Radziwiłłów line. Following the demands of the Entente, the Polish offensive was halted and the troops of General Haller adopted defensive positions. On June 8, 1919, the Ukrainian forces started a counter-offensive, but after three weeks were stopped near Gniła Lipa and upper Styr river. Then the Polish commanders decided to ignore the Entente demands and started yet another offensive on June 28, that pushed the Ukrainians back to the line of the river Zbrucz. On July 17 a ceasefire was signed. On November 21, 1919, the Highest Council of the Paris Peace Conference granted the Eastern Galicia to Poland for a period of 25 years, after which a plebiscite was to be held there. On April 21, 1920, Józef Piłsudski and Semen Petlura signed a Polish-Ukrainian alliance, in which Poland promised the Ukrainian People's Republic the military help in the Kiev Offensive against the Red Army in exchange for the acceptance of Polish-Ukrainian border on the river Zbrucz.