Well if history is a guide WWW 3 will be caused by a tragic miscalculation that inflames a world experiencing profound economic and structural upheaval. I see the situation today having many scary similarities to the world in 1912 and 1934. Will the spark be a Chinese military adventure to distract from domestic economic troubles that goes terribly wrong, or something else ?
Personally I predict WW3 will start with an exchange of Nuclear missiles between Pakistan and India.......