1919 battle of the Polish-Ukrainian War
The Chortkiv offensive (Ukrainian: "Чортківська офензива, Чортків," Polish: "Ofensywa czortkowska, Czortków;" 7–28 June 1919) also known as the June offensive, was one of the most successful counteroffensive military operations of the Ukrainian Galician Army (UHA) against the Polish Army during the Polish-Ukrainian War in 1918–1919. Around 19,000–20,000 Ukrainians pushed back the Poles along the entire front of the 39,000-strong Polish Army, reinforced by the French Third Republic. 
Goals.
The Ukrainian goal of the offensive was to push the Polish Army back to the Zolota Lypa River to improve the morale of the local Ukrainians and the UHA, as well as to provide a defensible area from which to mobilize a larger force and push the Poles back past Lviv (known as Lwów in Polish), Przemyśl, Chełm, Lublin and other territories claimed by the West Ukrainian People's Republic after 1 November 1918.
Ukrainian offensive.
On 8 June 1919, the Ukrainian Galician Army, numbering around 19,000–20,000 men, assaulted Chortkiv, forcing the Poles to retreat to the Holohory–Peremyshliany–Bukachivtsi line. Under the command of general Oleksandr Hrekov, the Ukrainian forces reached Lviv, the main urban center of Eastern Galicia and proclaimed capital of the West Ukrainian People's Republic, although they were unable to capture it.
The Ukrainian forces also gained victories at Yahilnytsia (7–8 June), Vyhnanka (8 June), Bilobozhnytsia (9–10 June), Kopychyntsi (10 June), Yazlovets (11–12 June), Buchach (12 June), Terebovlya (12–13 June), Monastyryska (13 June), Ternopil (14 June), Kozova (14 June), Nyzhniv (14–15 June), Pidhaytsi (15–16 June), Berezhany (20–21 June). However, despite the town of Berezhany falling into Ukrainian hands, the Polish forces were able to mount an orderly retreat and prevent the Ukrainian forces from developing the offensive further, denying them ammunition stockpiles (which were badly needed by the Ukrainians) and slowing the momentum of the offensive considerably. On June 23rd, the Polish forces successfully attacked Holohory, pushing the Ukrainians back beyond the Zolota Lypa river. 
Polish counterattack.
As the Ukrainian Galician Army suffered from a general lack of ammunition, on 28 June the better equipped and numerically superior Polish forces broke through the Ukrainian lines at Janczyn and forced them to retreat and evacuate across the Zbruch River. Due to the support of the French Third Republic in the war, the Polish Army was able to assemble a much larger and more powerful force of around 39,000–40,000 men against the Ukrainian Galician Army, and the Ukrainians were forced to return to their previous positions and abandon the cities and villages they had captured during their recent offensive. Eventually, the Ukrainian forces were forced back towards the Dnipro Valley, which was then controlled by the Ukrainian People's Republic, another nascent Ukrainian state. 
Aftermath.
The collapse of the Ukrainian lines at Janczyn forced the Ukrainian forces to retreat from Galicia, and, on July 16, 1919, the government and armed forces of the West Ukrainian People's Republic moved to the territory of Ukrainian People's Republic further east. Despite the operational and strategic victory of Poland, the initial tactical victories of the numerically inferior Ukrainian forces against the comparatively better equipped Polish army had a profoundly positive effect on Ukrainian morale, both in Galicia and elsewhere, cementing the offensive as an example of Ukrainian military victories during the Ukrainian War of Independence.
Notes.
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