Because so much has been written about World War I and II, it is helpful to have a search strategy that focuses more specifically on the area of German involvement that you need.
Some useful Library of Congress subject headings are:
World War, 1914-1918, Causes
Hitler, Adolf, 1889-1945
Nazis -- History.
Americans -- Germany -- Biography.
World War, 1939-1945 -- Personal narratives, German.
World War, 1939-1945 -- Social aspects -- Germany.
Germany -- Politics and government -- 1918-1933
Germany -- Social conditions -- 1933-1945
Note that you can mix and match many subheadings such as "SOCIAL CONDITIONS" or "HISTORY" with many main headings.
Want to be sure that your Library Catalog search finds only facts? Use -FICTION in the Subject Keyword box.