Panetta believes there is a strong likelihood that Israel will strike Iran in April, May or June - before Iran enters what Israelis described as a "zone of immunity" to commence building a nuclear bomb. Very soon, the Israelis fear, the Iranians will have stored enough enriched uranium in deep underground facilities to make a weapon - and only the United States could then stop them militarily.
Israeli Prime Minister Benjamin Netanyahu doesn't want to leave the fate of Israel dependent on American action, which would be triggered by intelligence that Iran is building a bomb, which it hasn't done yet.
Israeli Defense Minister Ehud Barak may have signaled the prospect of an Israeli attack soon when he asked last month to postpone a planned U.S.-Israel military exercise that would culminate in a live-fire phase in May. Barak apologized that Israel couldn't devote the resources to the annual exercise this spring.
President Obama and Panetta are said to have cautioned the Israelis that the United States opposes an attack, believing that it would derail an increasingly successful international economic sanctions program and other non-military efforts to stop Iran from crossing the threshold. But the White House hasn't yet decided precisely how the United States would respond if the Israelis do attack.
"You stay to the side, and let us do it," one Israeli official is said to have told the US. This is President Obama's preference anyway, although if Iran is stupid enough to attack US assets, the administration appears more than willing to strike back:
The Obama administration is conducting intense discussions about what an Israeli attack would mean for the United States: whether Iran would target U.S. ships in the region or try to close the Strait of Hormuz; and what effect the conflict and a likely spike in oil prices would have on the fragile global economy.
The administration appears to favor staying out of the conflict unless Iran hits U.S. assets, which would trigger a strong U.S. response.
It is unrealistic to expect Iran won't hit back at us. No denials from the US will be credible in Iran's eyes - especially given their domestic political position that Israel and America are in cahoots in their efforts to destroy the Islamic state. If Iran hits back it will almost certainly be at both the US and Israel.
No doubt we will continue to warn Israel against attacking Iran. But is there anything we might do to halt the march to war?
U.S. officials see two possible ways to dissuade the Israelis from such an attack: Tehran could finally open serious negotiations for a formula to verifiably guarantee that its nuclear program will remain a civilian one; or the United States could step up its covert actions to degrade the program so much that Israelis would decide that military action wasn't necessary.
This also, is unrealistic. Tehran will never allow the kind of intrusive inspections that would satisfy the US, and beyond that, covert actions are not going to get rid of the uranium already enriched.
By all reports, Prime Minister Netanyahu has not made a final decision. When he does, the American response will not be very important in his calculus. What does that say about American-Israeli relations at this critical juncture?