What I think on the matter is that the graphs being interpreted this way is a very simplistic aproach.
I think what the graphs show in both Slavic - German context that you mentioned as well as V13 L283 context is a positive correlation score (to be tested). What this means in my opinion is not a foregone conclusion. Sure the various wars and movements could be the culprit, it could also be random variations of the same general pattern.
What I think it is more likely to show is the ecosystem/economic busts and booms of the past for specific regions as a whole. Humans with bronze swords can only do so much killing, lack of grain, drought, disease though is a much better explanation for why there is correlation between inhabitants of Central Europe between themselves, and of the Balkans between themselves.
Sure tribe raids tribe type of thing can have an impact, and an empire resettling hundreds of thousands of people like the Romans did with Dalmatia can have a larger impact, but I think the underlying driver is actually ecosystem / economic drivers. Keep in mind the Illyrian revolt was economic driven to begin with. The much more prosperous Southern Illyrians did not revolt, as they had little reason too, being one of the more prosperous regions along with Anatolia, Egypt and mainland Italy. Bit hard to make war without a belly full, or while fighting smallpox.
Ecosystem disaster + Economic bust --> Demographic change --> Power Vacuum --> Economic incentive --> Waring tribes is my explanation
and rather not
Waring tribes for fun --> Demographic change + Economic bust
We're talking about patrilineages. R-L2 grew massively with Tumulus while much of the Carpatho-Balkan sphere got just slaughtered and the demography collapsed. But the reason was indeed that they killed and drove out so many people from Pannonia. They just grabbed the land they liked, the resources they liked and lived from their herds and made children with some of the women they could grab along the way. That's the story of the R-L2 expansion with Tumulus culture and Middle Danubian Urnfielders, from the destructive Koszider horizon onwards, which destroyed whole people like the Transdanubian Encrusted Pottery people, which lived there before for many centuries and were one of the dominant groups of the region - before.
The only thing one side needs is having the edge in a specific respect, technology, tactics, ideology, demography and the will to conquer, for whatever reason. Could be because they were pushed too, but could be the sheer opportunity to grab something because of superiour means.
In the cases we are talking about, these patrilinear founder effects, are practically always connected to some clans expanding at the expense of others. You see it with E-V13 and J-L283 before, even being themselves from the Alpine-Carpatho-Balkan sphere, doesn't matter, they replaced other lineages on the way which now are very rare or non-existent.
The boom : bust cycles are real, but oftentimes if two boom, one booms more, if two bust, one gets busted more. It is very rarely about the absolute impact, but rather the relative. Like if in a time of catastrophy one side does just good enough, like the J-L283 under Roman rule, they got an edge over a people which obviously suffered much more, like the E-V13 lineages did.
But such a discrepancy can't be just neglected as being of no importance, it is very imporant, especially if having such a magnitude and duration.
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