The Romans landed in Britain in 43 AD and their colonization of that island brought in people from all across the empire of all races Rome could muster.
Bernard Cornwell writes in his second Book of the Arthur Series:
"…He closed his eyes. ‘Such stupidity!’ He dropped his hands and opened his eyes. ‘I believe in the Gods, Derfel, but do the Gods believe in Britain? This isn’t the old
Britain,’ he said vehemently.
'Maybe once we were a people of one blood, but now? The Romans brought men from every corner of the world! Sarmatians, Libyans, Gauls, Numidians, Greeks! Their blood is mingled with ours, just as it seethes with Roman blood and mixes now with Saxon blood. We are what we are, Derfel, not what we once were. We have a hundred Gods now, not just the old Gods, and we cannot turn the years back, not even with the Cauldron and every Treasure of Britain.’''
Where did this multiethnic society the Romans build in Britain go?
Into a hole in the ground, of course.
Every time they dig up a Roman graveyard in Britain they find the bones of people from all over the empire and beyond. They even dug up a Chinese guy’s skeleton once. None of these societies is sustainable and it doesn't matter if it’s 50, 100 or 500 years, one day they go all into the ditch. The only difference will be if another invader just genocides them all, or the last of them dies lonely of rachitis.
Masculine Axis: A Meditation on Manhood and Heroism
“Is there any thing whereof it may be said, See, this is new? it hath been already of old time, which was before us” – Ecclesiastes 1:10
California is starting to resemble this already: a few densely populated areas full of everyone in the world and a trash-strewn wilderness everywhere else. The roads and infrastructure are crumbling. Like Vox Day says, every multi-culture eventually becomes a monoculture one-way or the other.