Of course there was war.
There was never not a war during this time. This war, however, was a little different.
First of all, seeing the success their island counterparts had, all the various tribes of Angles and Saxons finally started to band together to form a sort-of oligarchy. Then, they decided to go to war on the Picts.
That was where Uther Pendragon drew the line.
He was cool at first with the Angles and Saxons getting their act together. The all the various tribes and kingdoms of Briton did that under his dad, and—aside from the whole Vortigern incident—things turned out really well for the Britons. There was peace and prosperity, and even a little bit of education and innovation.
And, seeing how well this worked out for him, Uther wanted the same sort of good things to happen to all of his immediate neighbors. After all, peace for them meant more prosperity for him, and who doesn’t want that?
Which brings me back to the war that the Angles and Saxons picked with the Picts. Basically, the Angles and Saxons wanted to conquer shit.
This wasn’t the first the first time they wanted that, and wouldn’t be the last.
On the other hand, the Britons and the Picts were totally cool with each other.
First of all, there was Duke Gorloise down in Cornwall, who was BFFs with young King Lot of Orkney.
Then there was Llewellyn of Rhos who was tight with Mungo of Fib.
Finally, there was Strathclyde which was a Pictish Kingdom signed on with Briton under King Constantine.
So, yeah, Uther wasn’t happy with the Angles and Saxons, and interfered in their war on behalf of his Pictish friends.
It wasn’t a pretty war, so I’ll skip to the end: Uther won, and became High King of the Britons, the Angles, and the Saxons.