These sources offer overviews of various historical time periods and themes. In addition to finding some basic historical details, you can use these sources to identify relevant readings to explore a particular case in greater depth.
If you're unsure of which historical cases might relate to the claim you're testing, you might want to browse these resources for ideas. Resources like CQ Researcher, the Political Handbook of the World, and historical reference books can give you basic information about conflicts and issues that might be interesting to analyze in your paper.