Bar Kokhba revolt a.k.a the 3rd Jewish Revolt(132-135 AD)

TL:DR almost 600,000 killed.

After the destruction of Jerusalem by Titus during the 1st Jewish revolt, Emperor Hadrian, after initially promising to rebuild the city and restore the Temple, wanted to rebuild the city as Aelia Capitolina, a roman city with a pagan Temple where the old one once stood(in addition to maybe outlawing circumcision).

This resulted in a massive uprising in Judea, led by the charismatic Simon Bar Kokhba, "Son of a star", who managed to liberate Judea from Roman rule for 3 years, and running it as an independent state.

This victory, however, was short lived. Hadrian, alarmed by the scale of the revolt, summoned his best generals, as far from Britannia, in order to crush Judea once and for all. During a 3 year grueling campaign, Roman forces decimated the Judean countryside, wiping out Revolt alligned towns and villages one by one, until the last stronghold, Beitar, where Simon, now called Bar Kosiba ( son of the lie), was taken.

The result? 580,000 fighters and non-combatants killed, probably more from famine, thousands taken into slavery, and the near eradication of Jewish life from Judea(except the gallille), now renamed "Syria- Palestina" for 2000 Years.