The Battle of Mons Graupius took place in AD 83 or AD 84. Agricola had sent his fleet ahead to panic the Caledonians, and, with light infantry reinforced with British auxiliaries, reached the site, which he found occupied by the enemy.
The allied auxiliary infantry, 8,000 in number, were in the centre, while 3,000 cavalry were at the flanks. The Roman Legionaires were in front of their camp wall, being kept in reserve. The Caledonian army under Calgacus was stationed on higher ground; its vanguard was on the level ground, but the other ranks rose in tiers, up the slope of the hill in a horseshoe formation.
After a brief exchange of missiles, Agricola ordered auxiliaries to close with the enemy. The Caledonians were pushed back up the hill. Those at the top attempted an outflanking movement, but were themselves outflanked by Roman cavalry. The Caledonians were then comprehensively routed and fled for the shelter of nearby woodland, but were relentlessly pursued by well-organised Roman units.
According to Tacitus, 10,000 Caledonian lives were lost at a cost of only 360 Romans.
The site of the battle is unknown but presumably lies in the Scottish Highlands, and Bennachie in Aberdeenshire on the border between the Highlands and the Lowlands has been suggested as the exact site. It has also been suggested that the decisive victory reported by Tacitus is an exaggeration, either by Tacitus himself, or by Agricola, for political reasons.