File this dispatch from Gareth Porter under "interesting if true":
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 trip was highly unusual, in that there was neither a press conference by the chairman nor any public statement by either side about the substance of his meetings with Israeli leaders. Even more remarkable, no leak about what he said to the Israelis has appeared in either U.S. or Israeli news media, indicating that both sides have regarded what Dempsey said as extremely sensitive.
The substance of Dempsey's warning to the Israelis has become known, however, to active and retired senior flag officers with connections to the JCS, according to a military source who got it from those officers.
Hmmm. So a "military source" heard this from "active and retired senior flag officers with connections to the JCS" who in turn learned about this from....someone. I'm not going to pretend that this is the most confidence-inspiring sourcing I've ever run into, but that doesn't mean it isn't true. It might be!