The mods below can't be detected and deactivated automatically but will cause serious issues when used with R.E.D.
CIVILIZATION 5 PATCH 1.0.3.279 DOWNLOAD INSTALL
The simpler way to download and install the files is using the Steam Workshop (see post #2), but a direct download version is also provided (see post #3). Data Files contains all the sound and graphic files used by the core mod, and is less often updated. WWII Edition is the core mod, it contains the code and defines for the new game rules, and is frequently updated. Both part are needed and can't be used separately Because of it's size, the mod is split in two parts.
A few mods are marked as authorized (see end of post). All other mods will be automatically deactivated to prevent potential incompatibilities.
Finish your current game before updating, savegame compatibility is not assured. The mod use a custom DLL, it's not Mac compatible.