Which is more threatening to U.S. security interests: the prospect of an Iran with nuclear weapons or the fallout from a U.S. (or Israeli) strike against Iran’s nuclear sites?

President Obama hopes he won’t have to make this choice. The administration is congratulating itself on having gotten Russia and China to vote last month for new U.N. sanctions on Iran. Congress followed up last week with even harsher unilateral economic sanctions. The hope is that these economic strictures will squeeze Tehran into freezing its nuclear program and resuming negotiations over the program’s future.

Yet, at a fascinating conference on Iran run by the U.S. Army Central Command and the Saban Center for Middle East Policy at the Brookings Institution, I heard deep skepticism from experts on Iran that sanctions would force Tehran to change its behavior — at least within the time it might take them to produce a nuclear weapon. CIA Director Leon Panetta puts that window at two years.

We can always hope for the best, and the Mideast is unpredictable. Yet I worry the administration may be boxing itself in to choosing between two very bad options: letting the ayatollahs get nukes or plunging into a third Middle East war.

The Obama team’s language on Iran’s nuclear program has gotten markedly tougher in recent months, obviously aimed at persuading Tehran to take U.S. warnings seriously. The words “all options are on the table” are repeated frequently. Secretary of State Hillary Rodham Clinton told a pro-Israel audience in March: “The United States is determined to prevent Iran from acquiring nuclear weapons.”

And lest Tehran think we might settle for containment, Secretary of Defense Robert Gates stated bluntly last month that the United States was not “prepared to even talk about containing a nuclear Iran.”

Having spoken so clearly, the administration will be in a bind if Tehran refuses to back down in the coming year. An impatient Israel might feel compelled to launch its own military attack if the United States hesitates. I was fascinated that Obama told an Israeli television reporter last week — during the visit of Israeli Prime Minister Benjamin Netanyahu — that the “relationship between Israel and the U.S. is sufficiently strong that neither of us try to surprise each other.”

Can he be sure?

What’s clear: If Iran holds tight, the president will have to reassess his Iran options in 2011. Several insights from the Iran conference speak to the daunting choices he will face.

First, despite Iranian rhetoric, these experts don’t think Iran’s regime has any intention of attacking Western countries — or Israel — with nuclear weapons, and bringing on a Shiite version of Armageddon. They believe Iran wants nuclear capability so it can pursue regional hegemony without fear of being attacked. (I’ve heard the same analysis in previous interviews with U.S. and Israeli officials.)

Yes, an empowered Iran — whose leadership has a dangerously overinflated sense of its potential reach — would be a threat. Iran gives (conventional) arms to Hezbollah and Hamas, which oppose Israel’s existence. And if Iran achieved nuclear capacity, the conferees feared (exaggeratedly, I believe) that Sunni Arab states might try to develop their own nuclear programs. But how does one measure those threats against the incalculable risks of attacking Tehran?

Given the dispersed nature of the Iranian nuclear program, there’s no guarantee that an attack would destroy it. Moreover, a war in the gulf could create a catastrophic economic crisis for a world just recovering from deep recession.

The U.S. military says it believes Iran’s Revolutionary Guard navy could not stop shipping in the gulf — including vital oil tankers — but could seriously impede it. Iran can mine the waters, block them with sunken ships, and fire cruise missiles from mobile launchers along its long shore. Such asymmetrical attacks are difficult to counter. Insurance costs and oil prices would soar.

Meanwhile, Iran’s proxies, Hezbollah and Hamas, would be rocketing Israel. And for those hoping a strike will cause the mullahcracy to fall, most Iran experts predict that, to the contrary, it will harden the regime.

So it isn’t so easy to conclude that one of these options trumps the other. Both are bad (which is why neither Obama nor Israel has yet made a choice). But a military strike could provoke a chain of unintended consequences even more disastrous than those that followed the Iraq war.

Some suggested at the conference that there might be a third option:

Let Iran retain a small enrichment program, under more intense international inspections, while forbidding the building of more centrifuges or the enrichment of uranium to a high level. The administration hasn’t ruled this out, although it’s unclear whether Iran is interested.

Before engaging in another Mideast war, a fourth option — containment — should also be considered squarely, despite its dangers.

While hoping sanctions will bite, the Obama team must consider multiple options, in case Iran stonewalls. Otherwise, I fear we’re heading down a path that could be disastrous for us and the entire Middle East.

— Trudy Rubin is a columnist and editorial board member for the Philadelphia Inquirer. trubin@phillynews.com