Twentieth-Century War and Conflict by 
Encyclopedia of World Trade: from Ancient Times to the Present by Listed are the Library of Congress class numbers for history-related subjects.
C Auxiliary Sciences of History
D History (General)
DA Great Britain
DAW Central Europe
DB Austria - Liechtenstein - Hungary - Czechoslovakia
DC France - Andorra - Monaco
DD Germany
DE Greco-Roman World
DF Greece
DG Italy - Malta
DH Low Countries - Benelux Countries
DJ Netherlands (Holland)
DJK Eastern Europe (General)
DK Russia. Soviet Union. Former Soviet Republics - Poland
DL Northern Europe. Scandinavia
DP Spain - Portugal
DQ Switzerland
DR Balkan Peninsula
DS Asia
DT Africa
DU Oceania (South Seas)
DX Romanies
E America, United States
F United States local history, British America (including Canada), Dutch America, French America, Latin America. Spanish America