1. Citations shows the careful work you have put into locating and exploring your sources.
2. Citations allows the reader to locate the work you used to shape your argument.
3. Citations allow you to acknowledge those authors who made possible particular aspects of your work.
4. Citations draw attention to the originality and legitimacy of your own ideas.
5. Failure to provide adequate citations constitutes plagiarism!
Turabian (Chicago Manual of Style, 16th ed.) is most commonly used to cite sources within History. However, check your syllabus or ask your professor to make sure you are using the expected citation format.
This link is especially useful to find out how to cite different types of material