Dear Senators Obama and McCain,

You are now engaged in a campaign debate over whether to talk with Iran. As I’m sure you both know, this is a political exercise that will have little relevance should you actually take office.

In the White House, you will find yourself spending more time on Iran than any other foreign policy issue. You’ll be reminded that the 1979 Iranian revolution is one of the signature events of modern history, akin to the 1917 Russian Revolution, and the US has never figured out how to deal with it.

You’ll gather your intelligence experts to help you understand the Iranian threat. They will tell you what they have told the current administration: We don’t know much about how the Iranian regime operates. There are at least four internal factions that seem to regulate each other, but we have little idea how.

We don’t understand the Iranians because the Iranians don’t understand themselves. The regime isn’t sure whether it is an ideological movement championing global jihad or whether it is merely a regional power seeking Middle East hegemony. Until the Iranians resolve this internal ambiguity, you can talk to them all you want, but they won’t be able to make a strategic shift or follow a more amenable path.

As you sit in the Oval Office contemplating how to engage Iran, you won’t be reliving the campaign debate about when to negotiate. You’ll be thinking about how to exert pressure.

You will develop newfound sympathy for your predecessors in the Bush administration. There are a hundred things they could have done differently, but the primary fault for the failure to contain Iran does not lie in Washington.

It lies first with the feckless international community. The United Nations has passed resolutions demanding an end to Iranian nuclear enrichment. Iran ignores them. The United Nations Security Council Resolution 1701 forbids the rearmament of Hezbollah in Lebanon. Iran rearmed them without consequence. Fault also lies with the terrified but nearly immobile Sunni world. It lies, too, with the axis of the avaricious.

The US and Europe try to organize economic sanctions against Iran, but the oil-rich Mahmoud Ahmadinejad was welcomed in Indonesia, and Iran signed a pipeline deal with India. The Shanghai Cooperation Organization, a security group headed by Russia and China, granted Iran observer status, while denying the US the same status in 2005.

This is the problem with multipolarity. When everybody is responsible, nobody is responsible. A rich rogue nation can flaunt the will of a disparate majority.

When you enter the Oval Office, Iran will still be on the march. Forget campaign declarations. You’ll try anything. If the Saudis want to launch talks with Tehran as they did in secret 18 months ago, you’ll quietly support them. If the Arab League wants to engage, you’ll spend weeks in the middle of it. If the Israelis think they can flip Syria away from Iran with new peace talks, you’ll accept their efforts. You won’t believe that any of it can work. But nobody knows what will.

You’ll spend most of your time not challenging Iran but merely trying to contain its arc of influence. You’ll spend hours, as the Bush administration has, wondering whether Syria’s Bashar al-Assad can be turned in a more Western direction. Nobody can make an educated guess about that because no outsider understands Assad’s mind.

You’ll enforce Executive Orders 13338 and 13441, which restrict the movement and financial transactions of top Syrian officials, but it’s not clear you can squeeze the Syrians more than the Iranians, who play a more violent game.

You’ll work ceaselessly, as the Bush administration has, to make sure the Lebanese government doesn’t dissolve. But Hezbollah’s military power is so formidable you won’t be able to negate its veto power over national policy.

You’ll find yourself consumed against your wishes by a multifront ideological war, with Iran pulling strings on one side and you scrambling to gather a moderate coalition on the other. You’ll feel constrained in every theater, and you’ll realize that you are not destined to play the victorious role.

Your job is to restrain Iran’s momentum until the fundamental correlation of forces can shift. For amid all the doleful news, there is a hopeful tide. Opinion is turning slowly against extremism. The über-analyst Dennis Ross says that he has noted it among the Palestinians. Michael Young writes that opinion is shifting against Hezbollah in Lebanon. Peter Bergen, Paul Cruickshank and Lawrence Wright have in their different ways written about the intellectual crisis afflicting Al Qaeda. It may not happen over the next four years, but as Ross has noted, where Islamists rule, they wear out their welcome.