What's the "edge" that lets them displace the existing peoples (some of them from other branches of the Indo-Europeans anyway) there?
Not sure what you mean
When two groups go to war one needs the edge to defeat the other. Greater numbers, better leadership, better weapons, better organisation, some other trick.....
Slavs outnumbering other people in Central Asia. That's the edge.
If it didn't work in OTL, when Central Asians nomads migrated west, I'm not sure numerical superior would do the Slavs good in this regard either.
The "edge" there, or so I've read, was -- apart from the previous inhabitants having already been weakened by other invaders -- the Slavs' possession of a better plough which let them farm more of the land and thus maintain a higher population there once they had arrived. That would allow for incremental expansion onto the steppes, as happened IOTL, but probably wouldn't be enough in itself to support conquest of the existing locals by a relatively quick migration.Why wouldn't it work? Many areas in europe that are now slavic speaking used to be romance/celtic speaking.
First and foremost- Slavs were NOT that numerous. Their numbers upon arriving to Europe (most likely Slav first European settlements were in modern Volhynia in Ukraine) were about few hundreds thousands strong- that's really not enough to beat others, but...
But Slavs can't migrate eastward unless they create a fleet and invade Japan or Americas. That's because Slavs "started" in central Asia on the border of Steppes, Persia and India. If they go further east, they either have to invade Tibet (rather not most pleasant place to live), or circumvent mountains and go to China. Handful of Slavs in China isn't very likely to win (although it's hard to say when exactly Slavs could start their migrations) and even if they do, they most likely assimilate to local culture.
I don't think they'd try it in the first place though- going east was most natural direction for tribes.
First and foremost- Slavs were NOT that numerous. Their numbers upon arriving to Europe (most likely Slav first European settlements were in modern Volhynia in Ukraine) were about few hundreds thousands strong- that's really not enough to beat others, but...
But Slavs can't migrate eastward unless they create a fleet and invade Japan or Americas. That's because Slavs "started" in central Asia on the border of Steppes, Persia and India. If they go further east, they either have to invade Tibet (rather not most pleasant place to live), or circumvent mountains and go to China. Handful of Slavs in China isn't very likely to win (although it's hard to say when exactly Slavs could start their migrations) and even if they do, they most likely assimilate to local culture.
I don't think they'd try it in the first place though- going east was most natural direction for tribes.
Most linguists believe the Slavs originated in approximately modern day Belarus/ N. Ukraine. So.... no. Unless you are mixing the Sarmatians up with the Slavs.
Anyways-- The reason the Slavs westward migration was so successful was timing. They arrived in the areas they did after the populations had largely been depleted by the Migration Period. The Vandals, the Goths, and many other powerful Germanic tribes who inhabited lands we now consider Slavic upped and invaded the Roman Empire, thus creating one hell of a vacuum.
This is why OTL happened. So, if we made a POD that perhaps lessened this; say the Huns didn't push westward but instead continued their invasion of Persia with more success, then there is no longer a vacuum in Europe to fill for the Slavs.
Now, how do we get them to move east onto the steppe? From what I understand, the Slavs weren't a steppe culture. They were sedentary and agrarians, not nomadic and pastoral. They would need to become this in order to survive on the steppe. Then, as they push east, they would have to dominate the Turkic, Mongolic, and Sarmatian (or what was left of them) tribes living there at the time. It's highly improbable, though not impossible. Stranger things have happened in history.
Anyways-- The reason the Slavs westward migration was so successful was timing. They arrived in the areas they did after the populations had largely been depleted by the Migration Period. The Vandals, the Goths, and many other powerful Germanic tribes who inhabited lands we now consider Slavic upped and invaded the Roman Empire, thus creating one hell of a vacuum.