Why is を used instead of が in the following sentence ending in 名詞+だ?

Question

I came across this sentence in my textbook:

日本人はなぜ欧米人を野蛮人だと思ったのでしょうか。

I would expect が or perhaps a contrastive は instead of を, but I can't understand why を is used given that in the clause 欧米人_野蛮だ野蛮人, the subject is clearly 野蛮人.

What is going on? maybe it has something to do with the fact that the sentence is used as a relative clause?

I found this Q&A that also deals with を instead of が, but it focuses only on sentences ending with adjectives, ~たい verbs and potential verbs, and it says nothing about sentences ending with 名詞 + だ, so it does not address my question.

Answer 1

The sentence you have in mind would be parsed this way:

日本人はなぜ『欧米人が野蛮人だ』と思ったのでしょうか。

The と is quoting the clause 欧米人が野蛮人だ.

On the other hand...

日本人はなぜ欧米人を野蛮人だと思ったのでしょうか。

Here, 欧米人 is the object of 思った. The structure is 「XXをYY(だ)と思う」"think of XX as YY", not 「欧米人を野蛮人だ」と思う.

「XXをYYと思う」 can be used this way:

子猫をかわいいと思う
自分を特別だと思う
山田君を正直者(だ)と思う

This structure can also be used with verbs 考える, みなす, etc:

努力をムダ(だ)と考える
同性間の結婚を合法とみなす

Answer 2

You can use が instead of を in the sentence.

• 日本人はなぜ欧米人が野蛮人だと思ったのでしょうか

which translates literally :

• Why did Japanese think that Westerners were barbarians?

whereas the sentence in the question translates literally:

• Why did Japanese consider Westerners as barbarians?

So arguably the answer is that it is a matter of the writer's choice of construction. Also the use of は is most likely to be excluded for avoiding a consecutive は-phrase. (E.g. see this answer for ～を～と construction.)

Alternative: generally there is a research topic called 格の交替 for which you can find tons of papers on the web (like the one in the liked question). I guess it is also possible to see the above two sentences is an instance of 格の交替, which is usually considered to occur by some difference of perspectives. But I don't really see any difference by use of が/を in the particular sentence.