Battle of Mons Graupius
The battle of Mons Graupius was the first major battle between the Romans and Picts on Scottish soil in either AD83 or 84. The battle was the conclusion of a campaign of intimidation, pacification and conquest conducted by the governor Agricola after 79AD which as was intended to release the mineral wealth of Scotland to the Roman Empire. The normal Roman tactic was to conduct a campaign of destruction which either resulted in a pitched battle that Roman discipline could win or in the submission of tribes. In this case neither happened as the northern tribes simply kept away from the Roman army. The local tribes did conduct a successful guerilla campaign attacking outposts and small contingents.
After over wintering further south, Agrical returned to Scotland in 83AD and finding the tribal federation near Mons Graupius, erected a camp nearby. After the tribes declined to attack him, he marched out and attacked next day and through good discipline and the use of his cavalry reserve, rolled up the line of battle killing a supposed 10000 tribesmen at a cost of 300 dead. After the battle, Agricola returned south and later left britain.
The location of Mons Graupius has never been established and remains a mystery to this day.
Resources: excellent site on the battle and possible locations here and another from University of Chicago