Secretary of State Hillary Clinton’s visit to Israel constitutes the latest in a series of intensive trips across the pond by American and Israeli political leaders, top officials, senior generals and security chiefs, at which the issue of how to stop Iran’s nuclear drive continues to dominate the agenda.

The degree of coordination between the US and Israel is extraordinarily high. The goal, of ensuring that Iran does not develop a nuclear weapons capability, is emphatically shared.

But there have always been understandable tensions between the allies — Israel is closer to Iran than the US, feels more acutely threatened by Iran than the US, and has less capacity to do military harm to Iran than does the US. And those tensions are only deepening as the months go by and Iran shows absolutely no sign of slowing its nuclear program. Quite the reverse: In its latest presentations to the P5+1 powers, Iran has spoken of its plans to build more nuclear reactors and a new uranium enrichment facility.

The way the US sees it, the sanctions pressure on Iran is growing, and the economic and political impact is palpable. The Iranian economy is in a tailspin, and Iran is having trouble selling its oil, the very life blood of the regime. While there is some cheating by international players, providing some oil sale avenues, this a relatively minor irritant, not a major flaw. No, Iran has not given ground at the various ministerial and technical levels during rounds of talks, but its very presence there underlines its discomfort. And it has conspicuously failed to break up the P5+1’s united front against it.

In the American view, there’s no knowing when, or whether, Iran may crack under the international pressure and agree to the constraints needed to safeguard its nuclear program. There’s no knowing, as the US sees it, whether the Iranians will persist in making the wrong decision. But if and when the US feels all else has failed, President Barack Obama has made abundantly clear that he will do whatever is necessary to thwart Iran’s nuclear weapons goals. He doesn’t want to use the military option. Nobody wants to use it. But it’s on the table.

Israel, however, reads much of the very same information very differently. It too sees the sanctions impacting Iran’s economy. It too sees the diplomacy continuing. But the only question that really matters here is whether the pressure is affecting Iran’s nuclear program. And Israel’s answer — with which the US cannot argue — is an emphatic, absolute no. However much the Iranians may be hurting, their nuclear program is in the rudest good health.

The warm public words expressed by both sides about US-Israeli coordination are heartfelt. Behind the scenes, the exchanges of information and assessment are truly open, serious and constructive. The US representative to the P5+1 talks, Under Secretary of State for Political Affairs Wendy Sherman, to give just one example, is in constant communication — in person, by phone and via video — with key Israeli officials including National Security Adviser Yaakov Amidror, seeking their input, consulting them on P5+1 tactics.

But there is one great unspoken secret at the complicated heart of this highly sensitive relationship between two true allies facing what, for one of them — the weaker and more immediately threatened one — is a potentially existential danger: There is no circumstance, absolutely no circumstance whatsoever, in which the United States will empathize with an Israeli decision to strike alone at Iran’s nuclear facilities.

No American official will come out and say this. No Israeli official will acknowledge it. But that is the case, notwithstanding Obama’s declared support for “Israel’s sovereign right to make its own decisions about what is required to meet its security needs.”

The United States subscribes to what might be described as the Meir Dagan assessment, the argument advanced by Israel’s outspoken former Mossad chief that a premature strike by Israel at Iran’s nuclear program would have limited practical impact — setting the Iranians back two or three years at best — while providing ostensible legitimization for Iran to subsequently redouble its efforts to get the bomb, in part because of the need for a greater capacity to deter demonstrably trigger-happy Israel.

And what, for the US, constitutes a “premature” strike by Israel? Well, any strike by Israel, actually.

How so? Because if the United States concludes that only military action can thwart Iran, then the president will order precisely such action, officials insist. And he will do so having made clear to the international community that diplomacy and sanctions had failed, and that there really was no alternative. And that were Iran to restart the program after a US-led military strike, the US would have no compunction about striking again if necessary, as often as was deemed necessary.

By definition, therefore, any resort to force before an American resort to force — by Israel or by anyone else, not that anyone else is considering it — would be “premature.” It would come before the US had given up hope that diplomacy and sanctions could render military action unnecessary. It would come before the US had made plain to the international community that there was no alternative. It would contradict US policy. In short, and at the risk of understatement, it would not go down well in Washington.

Neither side rules out the possibility of an Israeli strike, although both feel that the level of nuanced coordination makes it less likely. Israeli officials, to a man — and they are all men — utterly prefer to see Iran’s program halted without military intervention.

But if it turns out that Israel’s red lines are plotted very differently from those of the Americans, how might the US respond? What would the US do and say if Israel, as Washington saw it, were to jump the gun? If Israel, believing that it had no alternative because its very existence was at stake, resorted to military intervention?

That’s a question American officials hope devoutly they will not have to answer. Make no mistake, it’s also a question that Israeli officials hope devoutly they won’t have to ask.