The Brusilov Offensive (Russian: Брусиловский прорыв Brusilovskiĭ proryv, literally: “Brusilov’s breakthrough”), also known as the “June Advance”, of June to September 1916 was the Russian Empire’s greatest feat of arms during World War I, and among the most lethal offensives in world history. Historian Graydon Tunstall called the Brusilov Offensive the worst crisis of World War I for Austria-Hungary and the Triple Entente’s greatest victory, but it came at a tremendous loss of life.
The offensive involved a major Russian attack against the armies of the Central Powers on the Eastern Front. Launched on 4 June 1916, it lasted until late September. It took place in an area of present-day western Ukraine, in the general vicinity of the towns of Lviv, Kovel, and Lutsk. The offensive takes its name after the commander in charge of the Southwestern Front of the Imperial Russian Army, General Aleksei Brusilov.
Russian Empire: 500,000 to 1,000,000 casualties
Austira-Hungary/Germany/Ottoman Empire: 962,000 to 1,337,000 casualties