So like if some indigenous people in what’s called America sailed to Europe in some canoes then claimed it for themselves; would that work?
As long as they moved in, created settlements, conquered, made treaties with the locals and hen over time became the dominate culture.
It is, after all what happened when the Indians arrived in Europe about 5000 BC.
About five groups left of the original inhabitants, the Finns, the Hungarians, Estonians, Basque and Sámi.
So it has been since the beginning and so it shall ever be.