The Iranian nuclear program continues to advance, despite unprecedented economic and diplomatic pressure by the United States and its international partners. According to the International Atomic Energy Agency (IAEA), Iranian production of enriched uranium has sharply expanded in recent months, while agency inspectors have been prevented from accessing sites and scientists. Key components of Iran's nuclear program are being dispersed and moved underground.

Some have suggested that if economic and diplomatic efforts fail, we should accept a nuclear-armed Iran and seek to contain it, much as we did with the Soviet Union during the Cold War. Others argue that Iran's illicit nuclear activities are primarily a problem for Israel and are not as much of a threat to the U.S.

Both assertions are profoundly wrong. The prospect of a nuclear-armed Iran is a threat to the entire world, including particularly the U.S., and its destabilizing consequences are not containable.

To begin with, Iran's nuclear ambitions are a mortal threat to the global nonproliferation regime. If Iran succeeds in acquiring a nuclear-weapons capability, neighboring Arab states will seek their own atomic arsenals. The Middle East will become a nuclear tinderbox, and the odds of nuclear material falling into the hands of rogue terrorists will dramatically increase.

Iran itself is already the No. 1 state sponsor of terrorism in the world. If it acquires a nuclear-weapons capability, its proxies—groups such as Lebanese Hezbollah and Shiite militias in Iraq that have the blood of hundreds of Americans on their hands—will become significantly more dangerous, because they could strike at us and our allies while being protected from retaliation by Tehran's nuclear umbrella.

A nuclear-armed Iran would also threaten the global economy by holding Middle Eastern oil supplies hostage. Recently, Iran's leaders threatened to close the Strait of Hormuz. If Iran tried to do so now, the U.S. and our allies, including those in the region, would have an overwhelming military advantage. But what if Tehran had nuclear missiles?

There is still an opportunity for the world to convince Iran's leaders to abandon their illicit nuclear activities peacefully. But in order to achieve such a diplomatic settlement, several steps are urgently needed.

First, it is imperative that the U.S. and its partners accelerate and expand economic pressure on Tehran. The only thing Iran's leaders value more than their nuclear ambitions is the survival of their regime. Consequently, sanctions must threaten the very existence of that regime in order to have a chance of stopping its illicit nuclear activities.

As importantly, however, we must put to rest any suspicion that in the end the United States will acquiesce to Iran's acquisition of a nuclear-weapons capability and adopt a strategy of containment.

For this reason, we introduced a bipartisan resolution last month that explicitly rules out a strategy of containment for Iran and reaffirms that the U.S. has a vital national interest in preventing Iran from acquiring a nuclear-weapons capability.

Our resolution is designed to firmly and unequivocally state that a policy of containing a nuclear-armed Iran is unacceptable. It does not dictate the administration's use of sanctions, nor does it authorize the use of force—our two primary methods of ensuring Iran's nuclear weapons program is stopped. But by sending an unambiguous message to the Iranians that we are prepared to do whatever necessary to stop them from acquiring a nuclear-weapons capability, we stand the best chance of avoiding a military conflict.

Some have asked why our resolution sets the goal of preventing Iran from acquiring a "nuclear weapons capability," rather than "nuclear weapons." The reason is that all of the destabilizing consequences of a nuclear-armed Iran will ensue as soon as Iranians have the components necessary for a weapon—and by then, it will be too late to stop them.

When some say that our red line is a "nuclear weapon," it suggests that anything short of a working bomb is acceptable. This is exactly the wrong message to send.

That is why the comprehensive sanctions legislation passed by Congress in 2010 and signed into law by President Obama identified the U.S. goal as preventing a "nuclear-weapons capability."

This is precisely the goal around which all of us—Democrats, Republicans and independents, Congress and the president, America and our allies—can and must now unite. Our bipartisan resolution is an expression of that necessary unity.

A version of this article appeared Mar. 9, 2012 in The Wall Street Journal, with the headline: Iran Can't Be Allowed Nuclear 'Capability'.