why isn't this sentence wrong?
This sentence appears in the book Grammar and Vocabulary for First Certificate by Luke Prodomou; it establishes that the sentences is correct. And I wonder why, since I consider that "feels is in its interests" is wrong, isn't the word "is" wrong there?

"The solution in Kyoto depends on what the United States, the most powerful nation on Earth, feels is in its interests."

Question

why isn't this sentence wrong?
This sentence appears in the book Grammar and Vocabulary for First Certificate by Luke Prodomou; it establishes that the sentences is correct. And I wonder why, since I consider that "feels is in its interests" is wrong, isn't the word "is" wrong there?

"The solution in Kyoto depends on what the United States, the most powerful nation on Earth, feels is in its interests."

Answer

To analyse this, first take out the inessential modifier "the most powerful nation on Earth" to yield "...depends on what the United States feels is in its interests". This is essentially a reported question corresponding to "What does the US feel is in its interests?", and the direct answer would start "What the US feels is in its interests is ...."  Does that "is" seem OK to you? It's the same one.

Answer

"The United States" may have a plural noun inside it, but it's actually singular: it's the name of a single country.

Here are some more examples of plural nouns referring to a single thing:
"The United States is a powerful country" (one country)
"5 hours is a long time to wait" (one period of time)
"The Beatles is my favourite band" (one musical group)
...if you used "are" in any of those sentences, the sentence would be very strange