Use keywords and synonyms:
Battle of Chickamauga - Braxton Bragg - Army of Tennessee - 1863
Abolitionists - American Anti-Slavery Society - Underground Railroad
You can broaden your search results by typing an asterisk symbol * at end of the root of a word. When you do this, the computer will search for alternative endings for the word you have typed.
For example 187* (or *70s) yields results for:
all years of the 1870s,
Activist, Activism, Activists, etc.