World War Zero is the slightly more polished version of Iron Storm. The game was never released in North America and unfortunately features only single player campagin mode.

The objectives are exactly the same as those in IS.
The weapons are the same, same locations, same characters, same in game videos, different name.

WWZ images taken from the game CD's
WWZ in game videos

-- Download the original game manuals. They read just like the IS manual.
World War Zero Manual English
World War Zero Manual German
World War Zero Manual French