Every nation having a nuclear arsenal has developed scenarios in which they would use those nukes. Among all those nuclear armed nations, Israel is the most likely to launch a nuclear first strike. Part of the reason is that Israel is small and an intense conventional attack could do Israel much harm. A larger part of the reason is that Obama and other American presidents have promised to back Israel no matter what. The largest part of the reason that Israel is most likely to launch a nuclear first strike is because Israel has gotten away with murder in the past and expects to continue as before.
EU's Mogherini in Tehran to talk nuclear deal, regional issues (Reuters)