If The House of the Wolfings was an admiring reconstruction of old Germanic clan-based society as a self-contained world, The Roots of the Mountains shows the ability of that society to revitalise others. The "others" in this case are another Germanic people; one settled in small towns and villages in the valleys at the foot of the mountains. They are a people in the process of losing their past: the old clan organization is becoming rigid - though individuals are still elected to positions, those individuals are always members of one house - and while there are still no classes (in a time without war the thralls have disappeared rather than turning into a caste of serfs) there is the beginning of a division of labour by peoples: the Burgdalers, living by weaving, smithying, and trade with the outside world; the Woodlanders, hunters and carpenters, "freemen, yet as regards the Dalemen well-nigh their servants"; and the Shepherds.
The heroic age is over for these peoples; weavers and traders cannot naturally break into verse as could the old Wolfings - this book is in prose, unless people chose to sing a song. Nor can they seriously believe in elves, trolls and dwarfs; the one person who does hovers on the edge of being a laughing stock.
Into this quiet, unromantic rural world come two outside forces: the descendants of the Sons of the Wolf themselves (Morris has now, disconcertingly, begun to translate all Germanic names) who have migrated west to the mountains, perhaps the Carpathians; and the Huns, or 'Dusky Men'. Depleted in numbers (many have gone to fight as mercenaries for the Romans), evicted from their settlement by the Huns, the Sons of the Wolf still remember Thiodulf and their heroic past - not only remember him, but may even be the ones who told the story of the House of the Wolfings (the teller of this story is later still, and comes from Christian times). Unlike the rural people of the other valleys they do not give up hope faced with the brutality of the Huns, but are acting as an unseen guard on the borders of civilization, blocking any further advance.
The eventual merging of the Sons of the Wolf and the Burgdalers comes about through individuals; the story, at its heart, is a five-way love story, so that once again a theme of the novel is the relationship of individual to society. But it is a relationship that is beginning to change; the Sons of the Wolf may be heroes, but they are also relics of the past, fading away, and Thiodulf's simple identification with his people - an identification even built into his name - will soon no longer be possible. Now there is always a suspicion that the interests of individuals and peoples may in fact be different. As on one level the story describes the welding of different peoples together through the love of individuals, so on another, it shows the beginning of the end of this possibility. As the hero asks his lover at the start:
Is it that thou will suffer me to wed thee and bed thee at last as mere payment for the help that I shall give to thee and thine?
The doubt continues almost to the end; and it is shared by all but one of the five lovers in the story - the only one whose love is unrequited.
The Roots of the Mountains seems to be the story that inspired the subplot of the Dunedain, wanderers of fading heroic past defending the frontiers of the Shire against the Orcs, and the loves of Aragorn, Eowyn, Faramir, and Arwen in Tolkien's Lord of the Rings. The spurned lover who goes to war as a man and eventually marries another, the woman who may (or may not) be one of those who 'live forever', the dutiful captain prepared to give all to defend his people - and others - from evil; all can be found in The Roots of the Mountains (of course many of the themes can also be traced back to real Germanic origins which both Tolkien and Morris were fascinated by, but the coincidences seem too many for this to be the only cause of the similarities).
In Tolkien's case the Orcs are, literally, both inhuman and instruments of evil; they are also ugly, malformed, and killed in large numbers without compassion. The description has led to accusations of racism (against whom is not clear). Morris describes his Huns as:
Short of stature, crooked legged, long armed, very strong for their size: with small blue eyes, snubbed-nosed, wide-mouthed, thin-lipped, very swarthy of skin, exceeding foul of favour.
makes them unable to breed with the Goths, and similarly has them killed in large numbers without compassion. He has as a result also been accused of racism:
There are, in short, no "half-breeds" to complicate the issue. In this convenient thesis one may well see a reflection of theories during the spread of the British Empire to prevent infiltration of the rulers by the ruled 
Quite apart from the improbability of this thesis applied to a man a large proportion of whose life at the time consisted of speeches and writings against the iniquities of the British Empire, both Tolkien and Morris's descriptions are clearly derived from a nearly contemporary Gothic historian:
Filimer, king of the Goths ... found among his people certain witches ... he expelled them from the midst of his race and compelled them to wander in solitary exile afar from his army. There the unclean spirits, who beheld them as they wandered through the wilderness, bestowed their embraces upon them and begat this savage race, which dwelt at first in the swamps -- a stunted, foul and puny tribe, scarcely human, and having no language save one which bore but slight resemblance to human speech. Such was the descent of the Huns who came to the country of the Goths... They made their foes flee in horror because their swarthy aspect was fearful, and they had, if I may call it so, a sort of shapeless lump, not a head, with pin-holes rather than eyes.
Jordanes, Origins and Deeds of the Goths 
But it is also clear that with the Huns Morris is beginning to move away from detailed historical reconstruction towards a more abstract symbolism. The Huns have:
No delight in life, no sweet days do they have for themselves, and they begrudge the delights of others therein. Therefore their thralls know no rest or solace; their reward of toil is many stripes, and the healing of their stripes grievous toil.
A projected sequel to the Roots of the Mountains, The Story of Desiderius, was never completed, and Morris wrote no more true historical novels; but "delight in life" and its enemies were soon to reappear in different forms in his fantasy novels.
 Goths and Huns: the rediscovery of the Northern Cultures in the nineteenth century, T.A.Shippey, 6th International Symposium on the Mediaeval Legacy, Odense 1981.
 See also Ammianus Marcellinus, History of Rome
Introduction by Graham Seaman, 16th April 2003. XHTML version created by Graham Seaman, derived from the text file prepared for Project Gutenberg from the 1896 Longmans, Green, and Co. edition by David Price.