Polish–Soviet War
Polish–Ukrainian War

On 18 October 1918, the Ukrainian National Council was formed in Eastern Galicia, still part of the Austro-Hungarian Empire; it was led by Yevhen Petrushevych. The establishment of a Ukrainian state there was proclaimed in November 1918; it had become known as the West Ukrainian People's Republic and it claimed Lwów as its capital. Because of Russia-related political considerations, the Ukrainian attempts failed to generate support of the Entente powers.

Key buildings in Lwów were seized by the Ukrainians on 31 October 1918. On 1 November, Polish residents of the city counterattacked and the Polish–Ukrainian War began. Lwów was under Polish control from 22 November. To Polish politicians, the Polish claim to Lwów and eastern Galicia was indisputable; in April 1919, the Legislative Sejm unanimously declared that all of Galicia should be annexed by Poland. In April to June 1919, the Polish Blue Army of General Józef Haller arrived from France. It consisted of over 67,000 well-equipped and highly trained soldiers. The Blue Army helped drive the Ukrainian forces east past the Zbruch River and decisively contributed to the outcome of the war. The West Ukrainian People's Republic was defeated by mid-July and eastern Galicia had come under Polish administration. The destruction of the West Ukrainian Republic confirmed the belief held by many Ukrainians that Poland was the main enemy of their nation.

From January 1919 fighting also took place in Volhynia, where the Poles faced the forces of the Ukrainian People's Republic led by Symon Petliura. The Polish offensive resulted in a takeover of the western part of the province. The Polish–Ukrainian warfare there was discontinued from late May, and in early September an armistice was signed.

On 21 November 1919, after contentious deliberations, the Allied Supreme War Council mandated Polish control over eastern Galicia for 25 years, with guarantees of autonomy for the Ukrainian population. The Conference of Ambassadors, which replaced the Supreme War Council, recognized the Polish claim to eastern Galicia in March 1923.
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