I think Israel (and, unlike GP, also Iran) are too small-time to start real global war. They're more the potential Bosnias or Serbias - small, belligerent countries that could drag their patrons into conflict.
The nuclear bombing of Iran by a state would almost certainly be enough to precipitate the complete collapse of the NPT, which is already wobbly (the latest Review Conference ended in acrimony).
Subsequent nuclear weapons projects in erstwhile NPT NNWS could easily trigger a few regional wars, and from there the escalation to a global conflict is certainly possible.
