I don't think we'll ever see another World War. Modern warfare has changed too much. It's no longer huge armies fighting it out, it's about quickly taking out the government and destroying communication between the leaders and troops. This change took place in the 1990s, after Vietnam and after the Russians invaded Afghanistan.
Look at what the US did to Iraq. Iraq got bitch smacked, and was defeated in thirty days or so. Same with with Israel - They woke up one morning and bitch smacked most of the Middle East, attacking ten countries including Egypt, Syria, and Jordan, and had them begging to surrender in less than six days.
I don't think the shit will hit the fan if Israel attacks Iran. It wouldn't be the first time that Israel has attacked suspected nuclear sites - once in Iraq and once in Syria. Both times the response was pretty much nothing. What would Iran do? Airstrikes against Israel? A missile attack? Highly unlikely because Israel would just fire back with more of same.
Here's a great article in the WSJ I just found about what Iran might do. Very interesting reading really.