From all appearances, Iran is a rational state, acting responsibly. 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." 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 choses 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 it against any contemplated surprise attack.
Australia to spend $64.9 billion on ships, submarines over 20 years (Reuters)
from the article: Mossad chief: Nuclear Iran not necessarily existential threat to Israel