Start with broad keywords.... like the time period....topic....person and then use the database tools to refine (limit) your results.
What are keywords?
Dates (1920s, Roaring Twenties, etc.)
People
Places
Events (September 11, 9/11, Pearl Harbor, etc.)
Topics
Use the terminology used by the experts, try synonyms. Remember results might be found under theater or theatre.
World History in Context US History in Context
It is possible to search both of these databases at the same time using the drop down selection as shown below.