Try online History encyclopedias for overviews about your topic -- they can either be specialized, like:
Concise Princeton Encyclopedia of American Political History
The Oxford Encyclopedia of Medieval Warfare and Military TechnologyOr they can cover wide, general areas:
A quick read with the right article in a scholarly encyclopedia on your topic will make your searches better and faster. Why? Simply because any professional or scholarly field has its own history, important names and specialized jargon. If you don't know them, then your online searching is much harder.
Encyclopedias:
Reader's Guide (Journal and Magazine Article Indexes):
Biographical Materials:
