From my good friend Gareth Porter at IPS:

Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey told Israeli leaders Jan. 20 that the United States would not participate in a war against Iran begun by Israel without prior agreement from Washington, according to accounts from well-placed senior military officers.

Dempsey's warning, conveyed to both Prime Minister Benjamin Netanyahu and Israeli Defence Minister Ehud Barak, represents the strongest move yet by President Barack Obama to deter an Israeli attack and ensure that the United States is not caught up in a regional conflagration with Iran.

But the Israeli government remains defiant about maintaining its freedom of action to make war on Iran, and it is counting on the influence of right-wing extremist views in U.S. politics to bring pressure to bear on Obama to fall into line with a possible Israeli attack during the election campaign this fall.

...The message carried by Dempsey was the first explicit statement to the Netanyahu government that the United States would not defend Israel if it attacked Iran unilaterally.

Gareth has a great track record on being right about Iran so, like, woah.