Should it be:
In nineteenth-century United States, many railroads were constructed.
In the nineteenth-century United States, many railroads were constructed.
Certainly if America were used in place of United States, then there’d be no definite article. But if I insist on saying United States, should there be one?
This question is different from the other. In my opinion, the latter sentence is somehow awkward and there is the possibility that the first sentence is 'correct'. If you think it obvious that the first sentence is wrong and the second sentence is correct, then please, instead of closing this question, answer it and explain why it is so obvious. Or why the other linked question clearly explains my question here.