Which section of a research paper should contain the main idea?

The main idea should be in the body paragraph. It would also depend on the topic you are discussing in the body paragraph.

It should be included in your introduction. It's normally your thesis.

On april 20, 1814, the dethroned emperor left france for the isle of  elba, where he was exiled under the terms of the treaty of fontainebleau.napoleon  would be allowed to ruleelba, which had 12,000 inhabitants. perhaps cruelly, the treaty allowed him to retain the title "emperor."