From Jerusalem, David Ignatius writes: 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.

As Halevy says, a war against Iran would be “an event that would affect the course of this century,” but when Ignatius says Netanyahu is prepared for Israel to go it alone, the columnist is merely parroting his sources.

If Israel really has the ability or intention to go it alone, what is it waiting for? Netanyahu’s red line of threshold capability has almost certainly already been reached. In spite of this, Israel threatens military action yet doesn’t act.

What Netanyahu can do is trigger a war that Americans must then fight. Israel’s war will then become America’s war for the simple reason that no one in Washington has the guts to let Israel do what it claims it can: go it alone.

When Halevy says that the threats of war are not a bluff, what he should really be saying is that Netanyahu is willing to start a war, but he’s not willing to accept responsibility for its consequences.