Many books and articles and more have been cataloged using a controlled vocabulary schema called the Library of Congress Subject Headings. In some cases, the LCSH is similar to or the same as keyword (or natural vocabulary) search terms or metadata (example: Soldiers). Wars have LC subject headings that are very different from keyword terms. Knowing and using both will give you the best chance of finding the best sources for your project. Keep a research journal of LCSH and keywords that work best. Note: some journal databases use controlled vocabulary that are not LCSH, so note these terms too.
World War (1914-1918).
Europe—Politics and government—1871-1918.
Germany—History—William II, 1888-1918.
World War (1914-1918)—United States (Germany, England, etc.).
More LC subject headings for World War I: http://libguides.lib.msu.edu/WWI-SH
A few examples: Great War, First World War, World War I, World War 1, World War One, WWI, WW1, Battle of the Argonne, 128th Army Infantry Regiment
REMEMBER: Keep a list of keywords that are working for your searches!