This also finds some confirmation in written sources. According to the PVL chronicle, Slavic tribes of the Radimichs and the Viatyches were not native East Slavs, but were recent immigrants to the region, and of of Lyakh (= Lechitic; Western Slavic) origin.
English translation of the PVL here: http://www.mgh-bibliothek.de/dokumente/a/a011458.pdf
And from wiki: https://en.wikipedia.org/wiki/Radimichs
https://en.wikipedia.org/wiki/Lechites#Lechitic_group
Also Constantine Porphyrogenitus mentioned "Lendzaninoi" (a West Slavic, Lechitic ethnonym) among the Dnipro river route Slavic tributaries to the Rus of Kyiv, locating them between the Kriviches and the Severians. He does not yet speak of Radimiches, but that's the correct area for them - so maybe those Lendzaninoi later became Radimiches (after their ruler called Radim as the PVL explains). The DAI discusses the situation of ca. year 952 AD, few decades after the beginning of the Slavic Wars of Henry the Fowler, and the battle of Lenzen (929 AD).According to Nestor the Chronicler, the tribe of Radimichs were Lachy (Lechitic) ... Due to some foreign invasion they moved to the East. (Original Russian text "радимичи же и вятичи — от рода ляхов. Были ведь два брата у ляхов — Радим, а другой — Вятко; и пришли и сели: Радим на Соже, и от него прозвались радимичи, а Вятко сел с родом своим по Оке, от него получили свое название вятичи.")
Also A. I. Kushniarevich (Russian Journal of Genetics: Applied Research, 2012, Vol. 2, No. 2, pp. 114-121) wrote:
"The migration processes of the later period could enhance the regional differences in the distribution of the studied molecular marker [sub-branch M458 of R1a haplogroup] in the population of modern Belarusians. In particular, a significantly high frequency of the R1a-M458 haplogroup in Ponemanye and Eastern Polesie regions of Belarus can reflect a movement of the tribal communities with genetic characteristics close to those in the local population into the Neman River and Pripiat River basins. Such communities could be for example Polabian Slavs, who left the territory between Elbe River and Oder River at the end of the First and beginning of the Second Millennia under the influence of the expansion of Germanic tribes [Saxons, etc.]. Toponymic parallels, as well as previous results of studies of linguistic and archaeological science, indicate the relationship between the Polabian Slavs and the population of modern Belarus (Jezowa M., 1962; Sedov, 1982; Perkhavko, 1983)."
And here Valentin Sedov's book: https://app.box.com/s/j5b27k8cvr4l415qcokb
A theory that Polabian Slavs emigrating eastward under Saxon pressure played a role in establishing the Polish State during the second quarter of the 10th century also exists in Polish historiography, although it has been criticized by e.g. Przemyslav Urbanczyk, who prefers to see the origins of the Piast dynasty in Great Moravia rather than Obodritia; as well as by Gerard Labuda who rejects all theories of external influence altogether, and claims that Poland was established as the result of internal development of local tribes. Urbanczyk wrote:
https://www.academia.edu/11888388/Origi ... st_dynasty
"Much less attention was paid to concepts which explain the emergence of the Piast state by migration from the west of some Obodrite warriors who allegedly escaped from the Saxon aggression (see criticism in G. Labuda 2002, p. 50)."
But I've checked G. Labuda's 2002 book as referenced by Urbanczyk, and there is nothing there, which criticizes specifically the "Polabian theory". Instead, Labuda 2002 criticizes all theories of foreign origins of the Piast dynasty altogether - including also this one. The only sentence referring in particular to this theory which can be found in Labuda's book, is this:
"I am sceptical of ideas that the Polish realm began with an invasion by Obodrite knights."
However, if those Obodrites were refugees fleeing eastward, then we should rather expect entire tribes or clans - men, women and children - not just military elites. And it was rather - as suggested by Belarusian scholars - an eastward migration of war-tormented peoples (with some groups reaching even as far east as Belarus), not necessarily an armed "invasion".