That would be the weather; the harsh winters were brutal.
And what the weather didn't do, the many diseases certainly did.
No, it wasn't so much the native Americans -- like you were probably thinking.
The best answer for this question would be:
Alvarez grappled with the issue of writing a novel about a dangerous subject.
A central idea focuses on the dominant impression of a story in which a general truth of the story is found within it.
He thinks it's a butiful thing apart of this world