Mad Max 2: The Road Warrior They never say it happened but you get the impression that it did.
Based on a quick read of the script opening, I believe that the premise was the destruction of all the oil infrastructure due to the US invasion of Iran. There could have been a nuclear war, but it wasn't global if there was, because the script makes a point of telling about how the machines went off and the people killed each other.
One thing I discovered just now, is that the leader of the group holding the oil well, was once the CEO of an oil company.
I'll have to watch again to see if they left that in the opening montage, or if there are any other clues. I just remember that there is a lot crammed in there, including footage from Mad Max.