Published by Oxford University Press, this compilation of over 350 reference works includes dictionaries, handbooks, and short encyclopedias in many areas of the humanities & social sciences, including history. Note that items vary in length from short dictionary definitions to 3000 word essays. Longer entries will often include a bibliography of selected references for further reading on a subject.
For example, Oxford Reference Online includes The Oxford Encyclopedia of American Social History: a two volume work covering historical topics from a societal point of view. Searching this source using "World War I" as a keyword phrase brings up a general article on the social impact of World War I on the United States, but also includes articles giving background and details on WWI-related topics from the general to the very specific, for example: "Home Front", "Daughters of the American Revolution" (and their war work), "German-Americans", "Aliens during Wartime", or "Universal Negro Improvement Association."
To gain a quick chronological overview of the war that will also link you to more extensive discussions and often to selected further readings as well, use the Historical Timelines feature in the left-hand frame. Take a look at the Timeline : First World War.
The editors of The Chicago Manual of Style provide a website to support use of their style guide. Although the actual text of the manual online can be seen only by subscribers, a Chicago-Style Citation Quick Guide on constructing some of the most often used types of citations is provided for free.
Purdue University's Online Writing Lab [OWL] makes a more detailed guide to Chicago style available on their website.
Since the Rhode Island College History Department uses the Chicago bibliographic citation style in a modified way, the final arbiter of correct usage is your professor.
The Chicago Manual of Style was not created to support the needs of historians specifically. And so, though it is hundreds of pages long, there are still some unusual items, especially primary sources, which Chicago may not address thoroughly or at all. Consider consulting Evidence explained : citing history sources from artifacts to cyberspace by Elizabeth Shown Mills in the Adams Library Reference Collection: Ref D5 .M55 2007 for examples of sources such as cemetery records, voter rolls, deeds, etc.