The Battle of Mons Graupius took place in AD83 or, though less likely, AD84. Julius Agricola, the Roman governor 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.

Even though the Romans were outnumbered in their campaign against the tribes of Britain, they had difficulties in getting their foes to face them in open battle. The Caledonians were the last to be subdued. After many years of avoiding the fight, the Caledonians were forced to join battle when the Romans marched on the main granaries of the Caledonians, just as they had been filled from the harvest. The Caledonians had no choice but to fight, or starve over the next winter. The Caledonian irregulars were no match for the discipline of the legions. It is estimated that a total of 20,000 Romans faced 30,000 Caledonian warriors, and a further assembly of wives and children.

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.

It is said that the Roman Legions took no part in the battle, being held in reserve throughout. The successful auxiliaries had been recruited from the Batavians. According to Tacitus, 10,000 Caledonian lives were lost at a cost of only 360 Romans. 20,000 Caledonians escaped and Roman scouts were unable to locate them the next morning.

Following this final battle, it was proclaimed that Agricola had finally subdued all the tribes of Britain.

The site of the battle is unknown but presumably lies in the Scottish Highlands. Since the 1970s the preferred location has been near the hill of Bennachie in Aberdeenshire on the border between the Highlands and the Lowlands. A book published in the summer of 2005 by Edinburgh University historian Dr James E. Fraser claims the battle happened much further south on the Gask Ridge not far from Perth. Historian Stan Wolfson has suggested it may have taken place in Sutherland. 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.