He travelled far and wide after the fall of Mona and learned many things from the Irish and Gaulish Druids. One of the things he learned was that the Roman and Celtic Pantheons were similar enough to be interchangeable and he went on many HeroQuests to enable himself to access the Roman magics. He also learned that the Romans would not be destroyed by magic alone, so he returned to Britain to gain allies.
Being half-Fairy, Merlin lived beyond a normal human lifespan. Being a druid, his life was extended further. Being in command of many powers, his life was very long and stretched throughout Roman Britain. This meant that he could have influence where many before did not.
When Merlin returned to Britain, the Romans had conquered all of southern and central Britain, leaving only Caledonia to pacify. They had chained the Celtic Gods and were trying to weaken them by identifying the deities with their own deities. This suited Merlin as he could then more easily HeroQuest to gain the new powers.
After a while, Merlin built up political contacts and helped one Marcus Ambrosius to become powerful in Britain. Marcus lead an attempt to become Emperor which was crushed and he was executed. However, his grandson was one who was destined to become Emperor, as prophesied by Merlin. This was Constantine the Great who came from Britain, aided by Merlin, and led his armies to become Emperor, only to make Christianity the State Religion and to weaken the other deities. Once again, this did not bother Merlin as he could access many of the powers of the Saints. However, it did weaken the pagan Celts whose temples were restricted and who were reduced n political power by not being Christian. Constantine then ignored Merlinís advice as "pagan" and turned to his own, Christian advisors.
Merlin continued to play with the politics in Britain, setting up a succession of Governors as King of Britain and causing a number of rebellions. This managed to weaken the Romans in Britain as they could not fight on many fronts. However, each time the Romans retained their hold on the land.
Finally, Merlin made alliances with the surrounding peoples. He had dealt with the Irish and Scots for many years and was a trusted and respected Druid in their eyes. He made travels into Scandinavia and Germany and spoke to the barbarians there, organising them into fighting bands able to attack Britain. Once he had finished, Merlin arranged for all the bands to attack Britain at the same time. This hammer blow was enough to force the Romans from Britain and the Legions left to fight Barbarians closer to home. Merlin had succeeded.
However, success has its price and that price was the loss of Britain.
With the Legions gone, who would defend Britain from the marauders? The Count of the Saxon Shore still had power and a navy but acted as a warlord. The local kings had small armies but were not organised enough to mount a solid defence. After many attacks, one warlord called Vortigern invited two Jutish kings Hengist and Horsa as mercenaries to fight the Saxon raiders. This was emulated by other kings and soon many mercenaries were settled in Britain. They began as simple mercenaries but then moved on to create kingdoms of their own. More and more invaders came and settled small kingdoms. Finally, Merlin decided to create a High King to drive out the invaders. He settled on Uther PenDragon, a warlord from the West and used his influence to make him High King. All his influence failed to make him a good king or even a successful warlord. Uther had some success but allowed his personal feelings to override his duties.
Eventually, Merlin had to accept that Uther would not be the King to unite Britain and decided to make Utherís unborn son King in his stead. We all know the story of the Sword in the Stone and how Arthur was conceived and born and what role Merlin played in his career.
Eventually, Merlin conceded that by forcing Rome from Britain he had doomed Britain to conquest by the foreigners. This news broke his will and allowed him to be tricked by Morgana into being defeated and bound into the Underworld. However, he still had a single card up his sleeve. He made sure that whereas Arthur would be defeated, his legend would live on and would be accepted by the Invaders as their own legend. In that way, Arthur became a Hero of the Angles and Saxons as well as the Celts he defended.