Today’s second glitch:
How it should look:
The once-peaceful water dwellers have suddenly declared war on a settlement that was their only true ally. No one has any idea why.
Why it should look that way:
Once-peaceful is functioning as a single compound adjective. (You don’t absolutely have to hyphenate these words, but it does make the meaning more clear.)
The predicate noun (subject of the sentence) is dwellers, which is plural, so the right possessive pronoun to go with that is their, not its.
Noone is not a real pronoun. (If seeing it as two words, no one, somehow causes you intense distress or whatever, hyphenate it as no-one, and readers will understand what you mean even though this is incorrect/nonstandard. Apparently the hyphenated version is “preferred” by the majority using UK English, but the Cambridge online dictionary doesn’t even list it as an option, nor is it listed as a “chiefly British” variation in the proint dictionary — Webster’s Encyclopedic Dictionary, 2002 edition — that I keep here in my office.)