Why bother with subject headings when one can do keyword searches in the online catalog?
It’s true that you can find sources on a topic by doing keyword searches. But if you limit yourself to keyword searching, you are likely to miss important material on your topic that uses other terms. If you only need two or three books, you can probably find what you need by doing keyword searches, but if you are doing historical research, you can’t afford to miss critical material on your topic. For a comprehensive subject search, search with subject headings as well as keywords.
A good way to identify subject headings for a topic is to do a keyword search in the online catalog using terms you think describe the topic and try to identify a few relevant books. Look at the full record for those books to see what subject headings were used, then do another search on those headings.
As a rule of thumb, use fairly broad headings, as well as the specific ones that describe your topic, in order to make sure you haven't inadvertently eliminated relevant material that is contained within works of larger scope. Most likely you will find multiple headings to describe your topic, and you should use all of them. You can narrow your search in the online catalog by combining subject headings (as a phrase) with keywords, using the “Advanced Search” option.
China -- Foreign relations -- United States
China -- Foreign relations -- 1949-1976
China -- Politics and government -- 1949-
China -- Politics and government -- 1949-1976
Hong Kong (China) -- Politics and government
Higher education and state -- China -- Hong Kong
Cold War -- Influence
Cold War -- Social aspects -- United States
Cold War in literature
Cold War in mass media
Cold War in motion pictures
Cold War in popular culture
Motion pictures -- Political aspects
Motion pictures -- Political aspects -- China.
Motion pictures -- Political aspects -- China -- Hong Kong
Motion pictures -- China
Motion pictures -- China -- History
Motion pictures -- China -- History -- 20th century
Motion pictures -- China -- Hong Kong 
Motion pictures -- China -- Hong Kong -- History