Causes of Invasion

The cause of Severus’ invasion of Caledonia (modern day Scotland) was a
In 195CE, Clodius Albinus, the Roman Governor of Britain, had led most of the British legions into Gaul during his revolt against Severus.
After defeating Albinus, Severus had sent the defeated army back to their posts, though the British legion suffered large casualties at the Battle of Lugdunum.
This left Hadrian’s Wall undermanned and made it easy for the Caledonians to raid into Roman Britain.
This led to a massive increase in raids and attacks on Roman Britain.

Invasion of 208CE

In 208, Severus arrived in Britain with around 40,000 men and marched north to Hadrian’s Wall.
Once at Hadrian’s Wall Severus initiated a massive rebuilding project which finally made the whole wall into stone from timber), and served as a barrier against more attacks.
Soon, Severus marched north and occupied all the land between Hadrian’s Wall and the Antonine Wall.
After completing the occupation Severus began another reconstruction project but this time on the Antonine Wall.

Invasion of Caledonia

In 209, Severus led his army north into the highlands and roughly marched the same way Agricola had invaded over 100 years before.
Severus suffered heavy casualties due to the guerrilla tactics used by the Caledonians.
To counter this, Severus began a plan of holding down all the territory he could by the reoccupation of many of Agricola’s old forts and devastating all the territory he couldn’t occupy.
Soon, ear of extinction through Roman genocide led many tribes to reach a peace agreement with Severus.
Peace talks failed and it looked as if the war would continue until all the tribes had submitted to Rome or been exterminated.

Withdrawal from Caledonia

In early 210CE, Severus’ son Caracalla led a punitive expedition north of the Antonine wall with the intention of killing everyone he came across and looting and burning everything of value.
The plan was for Severus to follow his son’s army and permanently occupy all of Caledonia.
In 210CE, Severus became ill and went to York to rest and recover. He kept getting worse until 4 February 211 when he died.
Caracalla then called off the war against the Caledonians and headed back to Rome to consolidate his power.

Aftermath

The Romans never campaigned deep into Caledonia again because they lacked spare forces to continue dealing with attacks as most manpower was redirected to secure other occupied lands.
They soon withdrew south permanently to Hadrian’s Wall.
The war also led to the reinforcing of the British frontier, which had been in dire need of reinforcements.
Invasion also led to the weakening of the various Caledonian tribes. It would take many years for them to recover their strength and begin raiding in strength.