From all appearances, Iran is a rational state, acting responsibly. Therefore, the US believes it should not go to war to stop Iran. Since it is desirable to not increase the number of states having nukes, the US will continue to help by "threats." But Israel hould know that there will no US action. And if Israel acts alone against Iran, the US will not intervene on her behalf. It will be a private Israeli affair. If Israel choses to reveal this to the world, too bad, because then Iran will probably nuclearize quickly.
PM: We'll surround ourselves with fences in defense against the wild beasts around us (Haaretz)
from the article: Israel, U.S. discuss triggers for military strike on Iran