The Cthulhu Wars by Kenneth Hite

An alternate-history campaign sourcebook that reimagines the twentieth century as a battleground between humanity and the rising forces of the Mythos, weaving real-world geopolitics, wartime technology and occult lore into a globe-spanning narrative; it supplies timelines, factions, scenarios, NPCs and mechanics for large-scale mythic warfare and covert operations so game masters can run pulp-to-apocalyptic campaigns in which nations, cults and investigators clash for the fate of civilization.