Why the reach was invaded? Do you mean originally? Read the lore or ask in the lore forum.
I'm not going to read all of the lore, if you have a point to make go ahead. When I have a point to make I don't tell people to go read, I tell them what I've read.
I just compare it to America. As much as I love us and our country, we did the same thing.
Everyone else did the same thing, just a matter of degree. It's still interesting how some type "might makes right" (which is true), while others try to justify this and that by mentioning some traditions, long lines, ancestors, "our land" and what not.
I guess its just harder to take a bunch of savages (the forsworn) seriously. I just don't feel sorry for those guys
That's often just the result of demonization and indoctrination (generally speaking). Since you mentioned US as an example - if you look back few decades even the Soviets were portrayed as Klingon-like savages/brutes, and they pretty much matched the West in technology. But in Skyrim terms, the Nords compared to Imperials or old Dwemer are also just that - savages. Imperials are more advanced civilization-wise than Nords, and yet people feel sorry for the Nords but not the Forsworn compared to Nords. I have been in Markath and Forsworn impressed me far more than Nords there. Then I look at wiki and think "hmm it says here I should dislike Forsworn and go kill them", but in reality after talking to most people in Markath you want to kill the Nords.
