Iran is becoming a foreign-policy problem of almost immeasurable proportions - from its nuclear-weapons brinkmanship to its feverish support of Islamic fundamentalism and international terrorism.

But Tehran's most proximate - and often overlooked - threat to American interests is its attempts to destabilize Iraq by supporting and fomenting its own insurgency against Coalition and Iraqi forces.

Tehran is seeking a hasty retreat by the United States and its partners that will leave a political and security vacuum that Iran can readily fill, dragging Iraq into its sphere of influence - or, perhaps, carving off southern Iraq to create an Iranian "super state."

Without question, Iranian encroachment on Iraq must be prevented at all costs.

Some Middle East experts don't buy this take on Iran's involvement in Iraq, especially its geopolitical intentions. Yet Tehran plainly has every reason to want to see the U.S.-led Coalition in Iraq fail.

First, since the 1979 revolution, the "Great Satan" has been Iran's No. 1 enemy. The radical regime found it bad enough having American forces in the region before the Afghan and Iraqi wars, much less having 150,000 cranky, battle-hardened GIs right next door.

Now, Tehran faces not only the prospects of (at least some) American forces being stationed long-term in the theater, a fundamental check on Iranian power, but also the possibility that Iraq and Afghanistan could become strong U.S. allies.

Second, Iran's rulers are deathly afraid that the freedoms taking root in Iraq/Afghanistan will highlight the Iranian revolution's abject political, economic and social failures to Iran's increasingly discontented "baby-boomers." Iran's people (60 percent are under the age of 30, born after the revolution) will look more and more at the political, economic and social freedoms enjoyed by Iraqis and Afghans and ask: "Why not us?"

Third, Iran is a Shia Persian country in a tough Sunni Arab neighborhood. Bringing southern Shia-majority Iraq under Iranian influence - or, even, via secession from Iraq or civil war, Iranian control - will neuter long-time enemy Iraq as a threat.

Absorbing southern Iraq would not only debilitate Baghdad by cutting off access to Persian Gulf seaports, it would significantly increase Iran's size, population and oil wealth, putting Tehran on a trajectory to regional dominance.

Iran has been slipping clerics, intelligence agents and paramilitary forces into Iraq and bankrolling sympathizers, political parties and militants since the spring 2003 invasion to bring Iraq under its sway - while doing its best to keep its fingerprints off its dirty dealings.

But seeing Coalition forces facing a tough insurgency, Iran evidently decided to seize the opportunity to advance its cause, upping the ante by changing its tactics from garnering influence to actively instigating insurgency against U.S.-Coalition forces - even Iraqis who might stand in the way.

You want proof? Well, Coalition forces recently intercepted a number of shipments of explosives being spirited across the border from Iran to Iraq. Experts believe that a new, more lethal-type of roadside bomb - capable of destroying armored vehicles - is based on an Iranian design often used in the past by Hezbollah against Israel.

Just last week, Defense Secretary Donald Rumsfeld, seemingly choking off a desire to be more direct, said: "It is true that weapons, clearly, unambiguously, from Iran have been found in Iraq." Another senior officer claimed that the new bombs are, "the most sophisticated and most lethal devices we've seen."

But it's more than just these new deadly explosives: The Iranian Revolutionary Guard Corps-directed component of the insurgency probably consists of several hundred Iranians and Iraqis as well as members of Lebanon's Iranian-backed, Shia terrorist group, Hezbollah.

Some analysts believe the Iranian paramilitaries and Iranian-supported militias are training insurgents in southern Iraq as well as in Iran. In addition, it's likely that Iranian-led insurgents are being prepped by Hezbollah guerillas in southern Lebanon and the Bekaa Valley.

Iranian behavior is increasingly troubling and problematic for U.S. national security and regional interests - an Iranian-directed insurgency in Iraq is just the latest example of Persian perfidy.

It's time to stop handling Iran with kid gloves, especially while Iranian hi-tech bombs deployed by Tehran-backed insurgents are killing Coalition and Iraqi forces and civilians, encouraging civil war and destabilizing the country.

It's time for an aggressive rollback strategy against the Iranian regime - to address its drive for nuclear weapons, its sponsorship of terror in Iraq and elsewhere, and its repressive rule at home. The strategy should embrace biting economic sanctions, aggressive covert action - and even surgical military strikes to protect American and Coalition forces and interests.

Peter Brookes is a Senior Fellow for National Security Affairs and Director of the Asian Studies Centre at The Heritage Foundation, and was former Deputy Assistant Secretary of Defense for Asian and Pacific Affairs in the Office of US Defense Secretary Donald Rumsfeld from 2001-2002.