I was imagining that this would be part of the original slavic migrations into Germany, particularly along the Elbe and into Schleswig-Holstein. From there, they spread in the north to the Weser and Aller, and then only gradually spread out through Frisia and elsewhere.
I'm not sure slavic people would be able to reach this far as quickly. I mean, chasing or dominating Saxons is possible in a first time, but the Franks are too much of a power (demographically by exemple) to have Slavic people in their direct "sphere of influence". Furthermore, these *Wends would be likely as you suggested terrestrial and fluvial threats, when Frisian relied mainly on coastal seafare.
It occurs to me that unlike the Norse, these *Wends would be in a much better position to interdict river traffic or raid down them. The development of trade and tolls when things settle down would be interesting.
I don't think Norse or Wends could actually interdict traffics. OTL, the raids were mainly parasitic and based on the fact trade existed and wasn't interdict.
It would be much more the Franks that, by using the OTL ways of protection (fortified bridges, creation of autonom march) would length the trade.
I wonder how the added danger to Western European coasts would alter trade or demography. Flanders is right there, even if it isn't really rich yet.
I advise you
Dark Ages Economics of Richard Hodges for that. It's really a must-have. Basically, Frisia was important economically by dominating North Sea trade until being crushed by early Carolingians. By neutralising Frisia earlier, you'll just create an earlier vaacum where Norse or even Anglo-Saxons could fill. Finally, I'm not sure it would have really different consequences than OTL
If the Franks get particularly stretched, they might turn more heavily toward missionary diplomacy to divide their enemies and gain tributary allies, even if the direct tribute is token.
How exactly even more? I mean, it was their main diplomacy (except sword diplomacy) OTL already.
The Saxons for example get kicked in the teeth ITTL, whereas they were rather restive historically.
They were divided, and it's not impossible that while *Wends would be sucessful in a first time to take some OTL Saxony parts, Franks would use this opportunity to stop Saxon raids in Austrasia, making them stuck between two threats.
In this case, they might be much more willing to submit to Frankish overlordship if it means military aid.
Maybe, but it would make
1) Saxon principalities dominated by Franks (like Bavaria, Thuringia) in Saxony
2) Would prevent the *Wends to reach Elbe river and make them under Frankish influence (like Odobrites or Lusatians under Carolingians).
Once Western Europe starts proving too expensive to raid/invade profitably, I'd imagine the *Wends would start converting for the trade and political benefits.
OTL, Norse made both.
Usual raiding party was : 1)Raid town 2)Go to another town 50km further, 3)Sell the loot there.
Raid is a booster for trade and political benefits : just look how Charlemagne managed to use the Ring's loot for stabilizating the empire.
Maybe ITTL Christianization occurs more as a coastal thing in the Baltic, driven by *Wend missionary activity?
Here, I think the Franks would have been still the missionary dominance in Germany (after all they are the main christian presence in Germany) and, as you noticed, as they use christianization as a diplomatic tool, they would be reluctant to let *Wends use it.
Now, in the case were Franks know crisis (dynastical, economical, etc), they wouldn't be in position to prevent Wends to turn missionaries but it would be less a Wend thing than church being more independent of the crown and acting of its own with the support of local princes.
Could you summarize it a bit?
Basically the military defense of the cities in Carolingia was assured by a cooperation between King/Emperor, Bishop and Count.
(Bishop and Count have to held, and King was supposed to break siege.)
But of course, the disputes, rivalities and game of power between them were certainly damaging the defense. Look at the siege of Paris where the defense against Norse was sucessful but where the rivalities between king and Count Eudes led to an agreement of Louis for letting Norse passing freely.
In another book "Le Monde Franc et les Vikings", the situation where Norse are integrated is more analysed. Basically, thanks to lands, wealth and society where they could have a certain position of power (rather than based on warchief skill and sucess of raids, always uncertain) they could use Norse against Norse.
Given that the *Wends would be better placed than the Vikings to raid down the Rhine and its tributaries, how might that affect the Frankish response?
Depending of the era I suppose. Now, as you related that to the earlier slavic migrations, I would think *Wends would take the place of Saxons there, so : counter-raids, raids, conquest, integration, desintegration, independent principality within Christiendom.
The rivers seem like a natural line of communications to hold the Frankish kingdom (or Empire) together, but if they're under greater threat ITTL, greater control will have to be yielded to local lords (or churchmen) in what would otherwise be the core of the realm. Frankish domains seemed loosely tied as it was; would they able to sustain the decentralization necessary?
I'm not sure they would be in that greater threat OTL. Again, if Wends are on Elbe that earlier, they would be like Saxons ITTL. A nuisance, rather preventing stability than lowering it.
Frankish domains were both loosely and firmly tied. As said earlier, the king was the only one able to gather enough men for a campaign, so counts and bishops needed him to break sieges or prevent raids, and of course distributing the loot amassed during the campaign.
Furthermore, with the Carolingian, we have sacred kings, with all the legitimity the Church could give to them.
Oh, I don't doubt that would happen with the Wends much as it did with the Vikings ITTL. Assimilation just might be a bit slowed by the fact that there are two such raiding cultures--meaning there are more such elites that have to be accommodated, and yet are still both distinct in language and culture from each other and the Frankish domains.
Franks are quite used to distinct language and cultures. I mean, during Pippin's reign you have : Franks, Romans, Goths, Bavarians, Thuringians, Bretons, etc...
The goal here isn't to integrate population but the elites. The strength of Franks there is their institution and the fact you don't need "popular" support but "royal" and land-based one.
Historically, not only Norse but Bretons, Vascons (that were both big raiders on their time) were integrated within the frankish domain, and used against raiders.
Part of the reason I even posted this was speculation as to the linguistic changes that might be involved in a greater slavic contribution to Western European languages, particularly "English". The language that develops in Frisia ITTL would also be a rather interesting blend--even if in later centuries the area speaks a Germanic language again, it'd be wildly divergent.
Well...
First, as said, if you crush Frisia, you let Anglo-Saxons or Scandinavian fill the vaacum : the slavic people didn't had the seafaring (not at the same extent) tradition and use than them. I doubt they will be able to have a direct influence on english.
Second : I would think that the Slavic language would be at least as influenced by Frankish, Romance and Latin than it would influence them. Furthermore, the slavic influence would be local, dialectal (as the Norse influence on French Norman).
It's a question of cultural influence, prestige and demography.
But the other reason was the potential in adding another seafaring tradition to Northern Europe, long before the Dutch or even the Hanse. The *Wends aren't as well placed as the Norse for voyages to Iceland and Greenland, but they are better placed to control medieval trade and river traffic in the long term. It's not like the Vikings never traded, either, and when the *Wends settle down they're apt to find trading at least as profitable as raiding. And really, what could be better than a medieval Slavic Netherlands*?
[ABE SIMPSON MODE : ON]
Well, crushing Frisia without seafaring tradition wouldn't allow *Wends to control sea roads, as Franks OTL didn't.
On the other hand, they could have an interesting role of fluvial trade between Francia and Baltic Sea and could even take part of theplace of Varangians had OTL.