No such a “reversal” this has been coming, quote; "“Softening-up” post 9/11: Change of policy and change of munitions?
Quote; “Continuity in relation to the Bush-Cheney era: President Obama has largely endorsed the doctrine of pre-emptive use of nuclear weapons formulated by the previous administration. Under the 2010 Nuclear Posture Review, the Obama administration confirmed “that it is reserving the right to use nuclear weapons against Iran” for its non-compliance with US demands regarding its alleged (nonexistent) nuclear weapons program. (U.S. Nuclear Option on Iran Linked to Israeli Attack Threat - IPS ipsnews.net, April 23, 2010). The Obama administration has also intimated that it would use nukes in the case of an Iranian response to an Israeli attack on Iran. (Ibid). Israel has also drawn up its own “secret plans” to bomb Iran with tactical nuclear weapons:
"Israeli military commanders believe conventional strikes may no longer be enough to annihilate increasingly well-defended enrichment facilities. Several have been built beneath at least 70ft of concrete and rock. However, the nuclear-tipped bunker-busters would be used only if a conventional attack was ruled out and if the United States declined to intervene, senior sources said.” (Revealed: Israel plans nuclear strike on Iran - Times Online, January 7, 2007)
Obama’s statements on the use of nuclear weapons against Iran and North Korea are consistent with post 9/11 US nuclear weapons doctrine, which allows for the use of tactical nuclear weapons in the conventional war theater.*
Through a propaganda campaign which has enlisted the support of “authoritative” nuclear scientists, mini-nukes are upheld as an instrument of peace, namely a means to combating “Islamic terrorism” and instating Western style “democracy” in Iran. The low-yield nukes have been cleared for “battlefield use”. They are slated to be used against Iran and Syria in the next stage of America’s “war on Terrorism” alongside conventional weapons.
“Administration officials argue that low-yield nuclear weapons are needed as a credible deterrent against rogue states. [Iran, Syria, North Korea] Their logic is that existing nuclear weapons are too destructive to be used except in a full-scale nuclear war. Potential enemies realize this, thus they do not consider the threat of nuclear retaliation to be credible. However, low-yield nuclear weapons are less destructive, thus might conceivably be used. That would make them more effective as a deterrent.” (Opponents Surprised By Elimination of Nuke Research Funds Defense News November 29, 2004)
The preferred nuclear weapon to be used against Iran are tactical nuclear weapons (Made in America), namely bunker buster bombs with nuclear warheads (e.g. B61.11), with an explosive capacity between one third to six times a Hiroshima bomb. The B61-11 is the “nuclear version” of the “conventional” BLU 113. or Guided Bomb Unit GBU-28. It can be delivered in much same way as the conventional bunker buster bomb. (See Michel Chossudovsky, CRG -- Getting used to the idea of double standards: The underlying maxim is "we will punish the crimes of our enemies and reward the crimes of our friends", see also Article Commentary: A Sustained Reaction - Bulletin of the Atomic Scientists) . While the US does not contemplate the use of strategic thermonuclear weapons against Iran, Israel’s nuclear arsenal is largely composed of thermonuclear bombs which are deployed and could be used in a war with Iran. Under Israel’s Jericho‐III missile system with a range between 4,800 km to 6,500 km, all Iran would be within reach." Go to: https://www.alainet.org/en/active/40327
for full and extensive article.
*Italics mine.
Arafel: Reading between the lines is not difficult here, clearly there have been those right at the top of successive U.S administrations who have assisted Israel in its acquisition of nuclear weaponry. The implication is that a Pentagon Paper wall-of-silence has more-or-less always existed with regard to Israel’s nuclear ambitions and capabilities (and probably like the Pentagon Papers if such should be revealed would elicit only feigned surprise from most of the U.S populace -and most of the rest of the World-).
As the U.S has spread the B-61 “bunker-buster” around Europe Israel’s stick-to-beat the Americans with has been the Israeli nuclear program, the supply of smaller U.S designed bombs has no doubt been intended to ensure both “correct” somewhat “limited” use by Israel and to limit the possibility of either failure or explosive accident (which is how the U.S has unofficially justified supplying them to the Israelis to the international community -the implication also being that these bunker-busting weapons are primarily considered as for use against Iran-). The Israelis may have calculated that given the assistance of its Saudi allies and the local “good neighbour” policy between themselves Egypt and Saudi Arabia a tactical nuclear strike against the Yemeni rebels (who are supported at least by the Iranian administration), was “justifiable”, especially so as the Iran Deal was (at the time of the alleged strike), just about to be signed. Israel is known for forcing the hand of the U.S by stepping beyond the accepted boundaries of international behaviour, perhaps flashing their nuclear armoury has been another example.

Profiling.
Explaining Israel’s aberrant behaviour is not that difficult either, spoiled children who are rewarded when they behave like mavericks develop an inflated opinion of themselves and they make mistakes. No matter how hard the Deep State attempts to obscure the use of tactical nukes by the “Israel ibn Saud” alliance (incl. indulging in the wholesale breach of the human rights of the people of Yemen by trapping them within a war zone the Powers that Be don’t want the prying eyes or open ears of aid agency workers or journalists to penetrate -an apparently somewhat desperate and panicked reaction-), the truth will out but let’s hope it does so before another and bigger theatre of war is opened in either Korea or Iran and the initial flash of the Yemen strike is obscured by the fog of a larger nuclear war.
There is however a more subtle but equally plausible narrative, clearly the opinion analysts and manufacturers (both inside and outside), of The State Dept. have been exercised by the questions (and this for some time); “Does the public (both at home and abroad), now consider a limited nuclear exchange (or the asymmetrical use of such weapons in specific circumstances), to be a different animal to that of a full strategic exchange?” …(and), … “Can we make the limited “battlefield” (a somewhat loose term as applied here), use of lower yield nuclear weapons acceptable?” So perhaps the PtB thought; “We’ll let the Israelis drop one of theirs on Yemen and see if anyone jumps!” Outlandish? Not really when one considers just how much time and energy has gone into both the continuing Korean War and preparing for possible conflict with Iran, to put it in cold economic terms; “It’s worth a lot more than are a few more dead or outraged civilians in one of the poorest countries in the world” (that’s “to them” of-course). Maybe it’s a bit of both though (as usual in-fact when it comes to U.S foreign policy), with the right-hand not really knowing what the left-hand is doing.
Politicking is just that, however, international law states that if the Saudis and Israelis dropped a tactical nuke on Yemen this was a criminal act and a case should be brought against them and heard under the fullest of international and public scrutiny." https://www.arafel.co.uk/2018/03/update-on-unofficial-nuclear.html