The debate over when (and whether) to set a "red line" for attacking Iran's nuclear program slipped toward farce Thursday when Israeli Prime Minister Benjamin Netanyahu took a marker and drew a thick red line across a cartoon-like drawing of an Iranian bomb.

The scene was Netanyahu's speech to the U.N. General Assembly. And the subject was serious: how to prevent the Iranian regime from producing nuclear weapons.

But the Israeli leader's constant demand - that Washington set red lines for Iran's nuclear program, which, if ignored, would trigger U.S. bomb strikes - has become counterproductive. His effort to corner the White House into endorsing Israel's red lines before November elections was unseemly.

And his hyping of red lines that the United States can't adopt - and Israel doesn't want to enforce - undermines his credibility with Tehran. Worst of all, Netanyahu may be boxing Israel into launching a premature war just to prove he meant what he said.

In his U.N. speech, Netanyahu insisted that only by stating clearly what Iran must not do would the ayatollahs take a military threat seriously. For Israel, this means denying Iran the capacity to enrich enough uranium to build a bomb.

Netanyahu's cartoon bomb was marked with black lines that indicated various stages of enriching uranium; his marker red line was drawn just below the third stage, which produces material suitable for a weapon.

The problem with on-stage red line is that it ignores the many reasons that a real-life red line is so risky - and so unwise.

For starters, even Israel hasn't been able to define its own red lines clearly. For months, the Israeli press has been filled with speculation that Bibi (as Netanyahu is known) would deliver an "October surprise" - meaning an Israeli attack before U.S. elections. But at the United Nations, Netanyahu set Israel's deadline as next spring or summer.

No doubt, that red line could change based on new developments - say, covert setbacks to Iran's program such as another computer virus. Or, say, Syrian President Bashar al-Assad falls, sharply slashing Iran's regional power.

So why commit yourself to a public deadline for war when you might not want to meet it?

Dan Halutz, former head of the Israeli military, recently told the Brookings Institution's Saban Forum, in warning against setting red lines: "The situation can change. You can't stick to a decision taken at a certain time; you have to make a real-time decision." If you do issue warnings, he added, "keep some uncertainties that leave confusion on the other side."

Second, no U.S. president can commit to start a war on another country's timeline, especially a vague one. Current White House policy is to prevent Iran from actually producing a weapon - a process that will take much longer than six months.

As President Obama rightly told 60 Minutes: "I think the American people wisely want their president to make a clear-eyed, sober assessment and not jump the gun when it comes to another military involvement after a decade of war."

Third, any U.S. president must design that policy in accordance with America's interests. He must calculate at what point the threat justifies military action. (My guess is that, if elected, Mitt Romney would roll back his commitment to back Bibi's red lines all the way.)

Washington's threat assessment differs from Israel's. Netanyahu's urgency stems from the belief that the ayatollahs seek Armageddon to expedite the return of their savior. Foul statements by President Mahmoud Ahmadinejad stoke such fears.

Yet, many U.S. (and Israeli) security experts think the Iranian regime is too rational to entertain the idea of nuking Israel. An attack on Israel would destroy two million Palestinians, along with Islam's third holiest mosque, and guarantee a second-strike nuclear attack on Tehran. It would destroy Iran's (already fading) aspirations to become leaders of all Muslims, including Sunnis.

Moreover, Ahmadinejad, the prime spouter of apocalyptic rhetoric, is so out of favor with the regime that his spokesman was arrested in Tehran the day he spoke at the U.N. His presidential term ends next spring and won't be extended.

If the threat is less urgent, and the ayatollahs are rational, more pressure may still produce results. Last week, an Israeli foreign ministry report contended that international sanctions on Iran are having a dramatic impact, causing a 50 percent decline in the volume of oil exports. The sharp decline in oil revenue is causing domestic unrest. These sanctions may yet push the Iranian regime to agree to concessions on uranium enrichment.

Finally, no U.S. leader can set red lines for a new war that ignore a cost-benefit calculation. As former Mossad chief Meir Dagan told the New Yorker's David Remnick on the risks of an Israeli strike on Tehran: "You have to take into consideration the following questions: What would be achieved? What about five minutes after? And what are the consequences of such an attack?"

Dagan believes a preemptive attack by Israel would be "reckless and irresponsible." It would ignite a regional war with a very uncertain ending (and send oil prices soaring). Moreover, it would only delay Iran's nuclear program and probably accelerate its drive for a bomb.

The risks entailed should also make any U.S. leader wary of a premature commitment to war while other options exist to curb Tehran. That quick swipe of a red marker on stage misreads the risks of setting red lines.

E-mail Trudy Rubin at