Background
In the year 84 AD the Roman general and governor of the province of Britannia Cnaeus Julius Agricola was engaged in his second northern campaign against the as yet unconquered tribes in the extreme north of the island of Britain.
Having already subdued the northern Brythonic tribes of the Selgovae, Novantae and Votadini that occupied the territories to the south of the Forth-Clyde line, Agricola now pushed his forces northwards, intent on completing the business of the conquest of Britain. What stood in his way was the loose confederation of tribes to whom the Romans had given the name of the Caledonii or Caledonians.
Faced with this invasion of their tribal lands, the Caledonians amassed an army of some 30,000 men under the command of one Calgacus; "one superior to the rest in valour and in birth" and somewhere in the Grampian mountains they decided to make their stand. (1)
Against them Agricola commanded a force of some 15,000, comprising the Legio IX Hispana as well as auxiliary forces but also (and including, as Tacitus informs us "some Britons of remarkable bravery"). Having located the enemy, Agricola placed his auxiliary infantry, some 8,000 in number at the centre with his cavalry of 3,000 posted on his wings. The legionary troops were left drawn up in front of the intrenched camp in reserve since as Tacitus puts it, "his victory would be vastly more glorious if won without the loss of Roman blood" (2)
The course of the battle
The battle began with an exchange of missiles, with little damage to either side, before Agricola ordered forward three Batavian and two Tungrian cohorts to engage the enemy. Here the Romans had the clear advantage as the Caledonians with their small shields and slashing swords were unsuited to close order fighting. Neither where their chariots able to make much impact due to the disclipline of the Roman troops and hampered as they were unevenness of the ground.
Part of the Caledonian army had been watching the fighting from adjacent hilltops and now seeing the tide of battle turn against them they descended from their vantage point to attack the rear of the Roman forces. But Agricola had held four squadrons of cavalry in reserve for just such an eventuality; was repulsed and their retreat turned into a rout.
As Tacitus describes it;
Then, indeed, the open plain presented an awful and hideous spectacle. Our men pursued, wounded, made prisoners of the fugitives only to slaughter them when others fell in their way. And now the enemy, as prompted by their various dispositions, fled in whole battalions with arms in their hands before a few pursuers, while some, who were unarmed, actually rushed to the front and gave themselves up to death. Everywhere there lay scattered arms, corpses, and mangled limbs, and the earth reeked with blood
The Romans were victorious; they killed around 10,000 of the enemy against the loss of only 360 of their own men.
Aftermath
Even allowing for a certain amount of exaggeration in the Roman account it demonstrates what a formidable killing machine the Roman army could be; their professional discipline was sufficient to achieve victory even in the heart of enemy territory against superior forces.
But it also demonstrates their limitations. The day after the battle the Romans went in search of the remainder of the Caledonian forces, intent, no doubt, on finishing the job that they had started. But they were nowhere to be found; the tribes had melted back into the mountains. The battle of Mons Graupius was a stunning and comprehensive victory for Roman arms, but in itself it was not sufficient to impose their will on Caledonia.
An enemy that would not fight, could not be defeated and it was clear that the conquest of Caledonia would require the commitment of significant military resources. As it turned out, events elsewhere in the empire were to frustrate any such ambitions.
NOTES
(1) The location of the actual scence of the battle is uncertain, but the lost likely candidate is somewhere in the vacinity of Stonehaven or Bennachie.
(2) The auxiliary troops, both cavalry an infantry, had been recruited from the ranks of the provincials of the empire, who would not have been, in the first century, Roman citizens. Hence letting the auxiliaries do most of the fighting (and therefore dying) was a 'good thing'.
See the Life of Cnaeus Julius Agricola by Tacitus located at www.fordham.edu/halsall/ancient/tacitus-agricola.html from which all quotations are drawn.