The library catalog and WorldCat use Library of Congress Subject Headings. The list of authorized subject headings is available online from the Library of Congress Classification/Subject Headings database. However, you can also start with keyword searches to gather some relevant titles, then look at their subject headings to discover the official terms to use.
Using these Library of Congress Subject Headings will aid your searches. Here are some examples of subject headings to use for the Revolutionary War and the Civil War:
Revolutionary War headings:
United States [or specific state name] -- History -- Revolution, 1775-1783 [browse for further subdivisions]
United States -- Foreign relations -- 1775-1783
[Names of battles, campaigns, seiges, etc.], e.g., Moore's Creek Bridge, Battle of, N.C., 1776
American loyalists
Evacuation Day, Boston, Mass., 1776
Minutemen (Militia)
Tea Tax (American colonies)
Writs of assistance
[place name] -- Burning by the British
[place name] -- British conquest
Civil War
United States [or specific state name] -- History -- Civil War, 1861-1865 [browse for further subdivisions]
United States -- Foreign relations -- 1861-1865
[Names of battles, campaigns, seiges, etc.], e.g., Bull Run, 1st Battle of, Va., 1863 [not Manassas]
Confederate States of America
Diplomatic and consular service, Confederate
Seccession -- Southern States
Propaganda, Confederate
Sherman's March through the Carolinas
Sherman's March to the Sea
Stone fleets
Unionists (United States Civil War)