Sumeria.
When the Akkadians took over they recontextualized the entire religion of Sumeria to better match their own. "Tiamat", a river-goddess was the cause of strife and pain, mother of monsters, and curses. The new "Head God" of the joint-pantheon is the Akkadian God who slew Tiamat in order to protect all Sumerians. "We killed your evil river god to save you. You're welcome. Now worship the new Ruler-God! Yaaaaay!"
Meanwhile Tiamat was just a river-spirit in the original Sumerian Religion. No evil or wickedness, just water flowing from one place to another.
But it was enough to cause big problems for the priesthood which were, like, the right hand of politics in the region.
Different cultures did it differently, of course.
This take is a little too reductionistic (as is your follow-up) and also wrong in a few key places, but I suspect that part of that rests in the subtle differences and blur between the various ancient Mesopotamian cultures. The push and pull of theistic beliefs is not always clear or so intentionally orchestrated as you make it out to be. Sometimes its gradual. Sometimes its voluntary. It's rarely so unilaterally clear-cut.
For starters, Tiamat wasn't a river goddess. Tiamat is a Babylonian goddess of the
sea and
salt waters, a representation of primordial chaos. Now I say Babylonian here because while Tiamat's name does appear in an earlier Akkadian insciption, we don't really see anything really resembling her character until the Babylonian Enuma Elish epic. In contrast, Abzu is the Sumero-Akkadian god of fresh water (e.g., rivers, lakes, springs, etc.) from underground sources and aquifers. There was no goddess named "Tiamat" in Sumerian mythology. There is a "Nammu" in Sumerian mythology who shares some superficial similarities to "Tiamat," but ancient Near Eastern scholars don't agree that she's necessarily the same character as "Tiamat" simply because they are both goddesses associated with the sea and creation myths, though this is not to say that Tiamat wasn't influenced by "Nammu."
Also, keep in mind that the epic of the Enuma Elish featuring Tiamat's death by Marduk does not necessarily represent an attempt by Akkadians to superimpose their beliefs on Sumerians. Bel Marduk was the patron deity of Babylon, a city-state founded by Amorites (a West Semitic people) rather than Akkadians (an East Semitic people), but the Amorites, or, rather, the Babylonians would speak and write in Akkadian. Also, there are a few texts where we find Ashur (patron deity of the Assyrian city of the same name) in the role of Marduk in these sort of mythic creation narratives. If anything, the Enuma Elish represents Babylonians imposing their beliefs on Akkadians (and possibly Assyrians) rather than Sumerians, who were already on the rapid cultural decline for centuries by the time of the Babylonian king Hammurabi (c. 1800), by some dating estimates of the Enuma Elish.
So this whole "We killed your evil river god to save you. You're welcome. Now worship the new Ruler-God! Yaaaaay!" seems far from factual and more like a personal narrative that you are projecting on ancient cultures.
Meanwhile when Rome conquered Anatolia in what is now Turkey, formerly held by those of the Altaic language group with their own religious beliefs, they wiped out the cultural traditions of the Altaic people except for Cybel. And the only reason she survived was a Prophecy that if the "Idaen Mother Goddess" was brought to Rome the invaders (Hannibal et al) would be expelled.
While there are some major problems with some of your other assertions, this snippet stood out to me as being particularly egregious and less "messy" to unravel.
When Rome conquered Anatolia (Asia Minor) in 130 BCE, there were no indigenous peoples in Anatolia who spoke an Altaic language. The Turkic migrations of Altaic-speakers into Anatolia is centuries off by the magnitude of likely 900-1000 years. Cybele was the mother goddess of the Phrygians, who were speakers of the
Indo-European Phrygian language, which was likely fairly-closely related to Greek. I vaguely recall that Cybele likely has an older origin with the Lydian peoples, but they spoke one of the Indo-European
Anatolian languages (e.g., Hittite, Luwic, Lydian, etc.). Also, by this point in time, Anatolia had been increasingly Hellenized (though not completely) by the various post-Alexander Hellenistic dynasties in the area. And the Phrygians were already worshipping a number of gods found in the Greek and Thracian pantheons, a number of whom the Romans had also obviously adopted to Hellenize their own pantheon. Worth noting is that 130 BCE is around 200 years after Alexander had cut the "Gordian knot" in the Phrygian temple dedicated to the Phrygian-Thracian sky god Sabazios, whom the Greeks had already associated with Zeus.
