Refined Celt fluff:
Britain was a tantalizing jewel of shore of Gaul, and it was only natural that Caesar should lust after it. So Brutus was sent, as a reward for his aid uncovering an assassination attempt. Soon after, the Celts, commanded by Boudica, He took a full legion, and invaded. But he was unprepared for what met him. His supplies dissipated, in small quantities, during the night. His men were harassed as they marched by blue-faced savages. More than one Man Machine was damaged by a falling tree. The men were quickly exhausted, and injured. And so when they arrived in a circle of standind stones, they were unprepared for what happened. The Machines stopped moving. Brutus set the smiths of Vulkan who had come to work at once. But the blue-faced demons burst from the trees, great clubs in hand. The soldiers drew their blades, but with mere mortal force, the blades glanced off the hides of the blue beasts as though they were cut from stone. the Romans were set upon, and defeated, even before the Treemen stepped out of the forest.
Boudica was pleased.
A mere week later, she launched an assault on northern Gaul. The Romans there were caught by surprise, not having heard of the defeat of Brutus, and the Celts successfully seized an additional legion of Man Machines and established a foothold on the continent.