From all appearances, Iran is a rational state, acting responsibly. As Mossad chief Pardo said, Iranian nukes do not pose an existential threat to Israel, and therefore to the West in general. Therefore, the US believes it should not go to war to stop Iran's nuclearization. Since it is desirable to not increase the number of states having nukes, the US will continue to help by "threats" and sanctions. But Israel should know that there will be no actual 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 chooses to reveal this to the world, too bad, because then Iran will probably nuclearize quickly. Israel is now developing a "second strike" capability (via submarines), and this is sufficient to protect her against any feared surprise nuclear attack.
Israeli teen lightly wounded from stone throwing in West Bank settlement of Kiryat Arba (Haaretz)