system
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human
[LOTR] How are Kingdoms (e.g. Gondor, Arnor, Angmar) established if people already live there?

It feels like Elendil just sailed from Numenor to Middle Earth and set up 2 kingdoms. Why would the people who already lived there simply accept new rulers, especially since he only had 9 ships of the Faithful? Similarly when the Witch King established Angmar, how did he convince the existing people to accept him, especially since he was still weak from Sauron's defeat and without an invasion army. How did Sauron, still weak, establish himself in Dol Goldur in Mirkwood when elves already live there? How did Arnor instantly reestablish itself after Sauron's defeat and have it's inhabitants automatically accept it. I have not heard of any major conflict in establishing these kingdoms.
gpt
Over the millennia, Numenoreans had emigrated into Middle Earth and established colonies. The Faithful claimed kingship over these Numenoreans (besides those south of the Anduin who refused). Not all people accepted their rule, being driven out North of Arnor or East of Gondor (Dunlandings, Black Numenoreans, etc.). 

Agmar was founded by the Witch-King with the help of the Black Numenoreans and was founded close to a massive orc stronghold (Gundabad) in a region known for being inhospitable to humans for the most part. 

Sauron established himself in Dol Guldur precisely because Elves didn’t live there; when the elf king Oropher who lived there was slain in the war of the Last Alliance, the elves retreated to Northern Mirkwood (Elves would continue to slowly retreat further north from encroaching dark creatures). 

No kingdoms replaced Arnor after it’s fall, the inhabitants for the most part accepted Aragorn’s claim because it was the Dunedain who had continued to protect the northern lands after the fall, and because there weren’t a lot of people living in lands that until recently had been known for being dangerous.