I do not think israel would" first use" an atomic weapon on an adversary unless they were NBC 'ed besides there are not too many military targets in iran that require a nuke and no sane power wants to go after a large city just for a body count . I trust the IDF is being VERY careful what nuclear facilities they are taking out as to not cause a man-made nuclear melt down radiation release across the gulf and farther .
They have a first use policy in the event of existential threats. They came very close to using them during the Yom Kippur war, to the point that planes were armed. These were tactical and were to blunt Egyptian and Syrian armored thrusts that were within an RCH of breaking out into Israel.
Mier order planes loaded. Nixon and Kissinger were given a veiled 'very serious consequences' if the war keeps going badly. This tipped the scales to getting US aid delivered... which was also a fisaco. Nixon and Kissinger did not want to supply aid as things swirling around Watergate and an anti-Israel faction in the cabinet and Congress (and Vietnam withdrawls) made it all sort of a challenge.
The Israel policy is called "The Samson Option."
This is a worthwhile read.
https://en.wikipedia.org/wiki/Samson_Option
Also, part of the reason that preempting Iran from exploding a nuke (which they were days/weeks away from doing) makes the Samson Option less necessary or unnecessary.
Once again... You Don't Mess with the Zohan!
Sirhr
