On June 4th, 1916, the Russian Empire opened up with heavy artillery fire on Austro-Hungarian defences across Galicia, modern day western Ukraine near the borders with Poland and Romania. This was followed up with Brusilov's 7th, 8th, 9th and 11th Armies attacking Stanislau (now Ivani-Frankivsk), Lutsk & Kovel, Czernowitz (now Chernivtsi) and Lemberg (now Lviv) respectively. What followed is one of the bloodiest offensives in history. This offensive wasn't about taking ground. The Entente allies had devised their strategy for 1916 during the Chantilly Conferences. Their plan was a Grand Allied Offensive that would attack the Central Powers from all sides simultaneously. Victory would be achieved through attrition, as Germany and Austro-Hungary would be unable to rest or reinforce their troops. However, the German attack on Verdun in February had caught the Entente unprepared. Not only would the Allied attacks need to destroy the Central Powers, they would now be needed to relieve the pressure on the French.
The opening of the Brusilov offensive was very successful, and the Russians recaptured much of Galicia that had been lost the previous year as Austro-Hungarian units were pushed back in a full retreat. But as casualties began to mount on both sides the Russian advance faltered. Lacking in supplies, reinforcements, and support from other generals on his flanks, Brusilovs offensive slowed and by August, the Germans and Austro-Hungarians managed to stop the advance. The Russians had already suffered many casualties, especially at Kovel, and continuing on was no longer possible.
While the Russians did manage to capture Stanislau, Lutsk, and Czernowitz, it came at a cost. Both the Russians and Austro-Hungarians suffered an estimated 1 to 1.5 million casualties each. The impact of the battle was far-reaching. Brusilov successfully achieved his goal to force the Central Powers to halt its attack on Verdun and send emergency forces east. His efforts also helped save the Italians who had themselves been hard-pressed since Austria-Hungary attacked them in May, and convinced Romania to join the Entente allies. Perhaps most importantly, it inflicted irreparable damage on the Austro-Hungarian army. Across 1916, the Central Powers had been placed under tremendous pressure and sustained huge casualties that were irreplaceable. But they were not knocked out of the war. Germany took control of the Eastern Front from Austria-Hungary, and the losses suffered by the Russians brought them to the brink of revolution

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