A popular new slogan making the rounds among government ministers here is that in dealing with Iran, Israel faces a decision between “bombing or the bomb.” In other words, if Israel doesn’t attack, Iran will eventually obtain nuclear weapons.

This stark choice sums up the mood among top officials of the government of Prime Minister Benjamin Netanyahu: It’s clear that Israel’s military option is still very much on the table, despite the success of economic sanctions in forcing Iran into negotiations.

“It’s not a bluff, they’re serious about it,” says Efraim Halevy, a former head of the Mossad, Israel’s intelligence service. A half-dozen other experts and officials made the same point in interviews last week: The world shouldn’t relax and assume that a showdown with Iran has been postponed until next year. Here, the alarm light is still flashing red.

Israeli leaders have been warning the Obama administration that the heat isn’t off for 2012. When a senior Israeli politician visited Washington recently and was advised that the mood was calmer than in the spring, the Israeli cautioned that the Netanyahu government hadn’t changed its position “one iota.”

The negotiations with Iran by the group of leading nations known as the “P5+1,” rather than easing Israel’s anxieties, may actually have deepened them. That’s not just because Netanyahu thinks the Iranians are stalling. He fears that even if negotiators won their demand that Iran stop enriching uranium to 20 percent and export its stockpile of fuel already enriched to that level, this would still leave more than 6,000 kilograms of low-enriched uranium that, within a year or less, could be augmented to bomb-grade material.

Netanyahu wants to turn back the Iranian nuclear clock, by shipping out all the enriched uranium. And if negotiations can’t achieve this, he may be ready to try by military means.

The numbers game on enrichment reveals a deeper difference: For President Obama, the trigger for military action would be a “breakout” decision by Iran’s supreme leader to go for a bomb, something he hasn’t yet done. For Netanyahu, the red line is preventing Iran from ever reaching “threshold” capability where it could contemplate a breakout. He isn’t comfortable with letting Tehran have the enrichment capability that could be used to make a bomb, even under a nominally peaceful program.

Netanyahu sees his country’s very existence at stake, and he’s prepared for Israel to go it alone because he’s unwilling to entrust the survival of the Jewish state to others. But some Israeli experts, including several key supporters of his government, don’t like this “existential” rhetoric warning of another Holocaust, arguing that it nullifies Israel’s defense capabilities and deterrence.

Though most members of Netanyahu’s government would probably support him, there are some subtle nuances of opinion. U.S. officials say Defense Minister Ehud Barak’s focus is stopping Iran before it enters a “zone of immunity” when it begins full operation of centrifuges buried under a mountain near Qom. Iran probably will enter this zone sometime later this year. As Israeli officials have put it, the deadline for action “is not a matter of weeks, but it’s not a matter of years, either.”

American officials think Barak may also be more willing than Netanyahu to accept a deal in which Iran retains some modest enrichment capability — and can save face by saying it hasn’t compromised its rights as a signatory of the non-proliferation treaty — but can’t accumulate enough material to make a bomb.

Some Israeli experts are skeptical about the “zone of immunity” timeline. They believe that no facility, even the hardened site at Qom, is invulnerable to a clever attack: Iran will have immunity only with an actual nuclear-weapons umbrella.

While I understand Netanyahu’s concerns, I think an Israeli attack could be counterproductive. It would shatter the international coalition against Iran, collapse the sanctions program when it is starting to bite and trigger consequences that cannot be predicted, especially during a time of sweeping change in the Middle East.

Before he rolls the dice, Netanyahu should recall the shattering experience of Menachem Begin, a prime minister no less devoted to Israel, who was haunted in his final days in office by the sense that his invasion of Lebanon in 1982, intended to protect Israel’s security, had been a mistake. The potential costs and benefits of an attack on Iran are unknowable, but for better or worse, it would be, as Halevy says, “an event that would affect the course of this century.”