There is no simple answer to this question. Several points should be considered:

• You need to tell the reader the source of your information when you think that the reader may doubt or be surprised by what you say. In this case you need to assure your reader that what you say in your paper is the result of serious research.
• Another reason to provide documentation or a citation for a source is to support a point that is important to your analysis of your theme. If your theme is "The Division of Korea at the End of World War II," one of the principal points about the division of Korea that you make in your paper may have to do with the motives of the United States and the Soviet Union, the two nations that temporarily occupied Korea when Japan surrendered. If so, then all of your examples of the motives of these two countries need to be documented. If you say that the United States occupied the southern half of Korea because it feared the expansion of the Soviet Union, then you need to tell the reader why you think this is so. The place in the paper where you make this explanation needs to be given a footnote or an endnote so that the reader will consider what you say seriously.
• You do not need to document everything you write. If you write something that is common knowledge or that almost every reader will agree with, you don't need to footnote or endnote it. If you write: "World War II ended in August 1945 after the United States bombed the Japanese cities of Hiroshima and Nagasaki," this common knowledge needs no documentation.