History Memes

Cannae in the mid-Republic - between 50,000 and 70,000 killed, though some of those would have been Italian allies. The Republic carries on the war despite those (and previous) losses for another ~15 years.

Carrhae in the Late Republic - 30,000 lost, most of them Romans. The Republic ends the war despite losing several Legion standards. This is particularly notable, as the war against Parthia was condemned by many in Rome as an unjust war, and a priest called the wrath of the gods down on Crassus (the commander and instigator of the war) before he left Rome. The legion system at this point is closer to the Empire, being largely volunteers, but still with a much lighter recruitment burden - troops serve less time, and are trained in a more ad hoc fashion.

Teutoburg Forest in the 1st century AD - ~15,000 Romans lost in three Legions, possibly a similar number of auxiliaries. Emperor Augustus resorts to conscription in several Legions (who would later cause trouble, on account of not wanting to be in the Legions) in response to this crisis. Rome embarks on a series of punitive campaigns to avenge their deaths and retrieve the Legion standards, but abandons the entire occupied area of Magna Germania in response to the loss.

Battle of Lugdunum in the 2nd century AD - Unknown total losses, but suggested to be in excess of ~30,000. Civil war bloodletting, so the casualties of both sides get counted as Roman losses. The pay for legionaries would be increased by ~50% after this, certainly to shore up the victor's popularity, but also likely as a recruitment inducement to replace losses.

After this, things head into the Crisis of the Third Century, and everything gets fucked. By the end of the Crisis, troops are no longer being paid regularly, they're just empowered to take whatever they need, and landlords are obligated to give the central government pseudo-serfs as conscripts for lifelong terms. Utter breakdown of the Imperial system for a gruesome and despotic replacement.