This illustration portrays pre-Marius reforms Republican Roman legionaries battling 2-3rd c. BC Celtic warriors. ~ the previous caption claimed this was during the Gallic Wars (58-50 BC). The siege towers make it seem this is the army of Caesar besieging Alessia, but the armour of both the Romans & Celts is from an earlier time.
Romans at the Battle of Cannae. Having recovered from their losses at Trebia (218 BC) and Lake Trasimene (217 BC), the Romans decided to engage Hannibal at Cannae, with roughly 86,000 Roman and allied troops. The Romans massed their heavy infantry in a deeper formation than usual while Hannibal utilized the double-envelopment tactic. This was so successful that the Roman army was effectively destroyed as a fighting force.