It's not a major war against Iran that is on the US agenda today, for which the US power elite lack the means and the US public have little appetite. It is economic sanctions, covert actions, and "democracy assistance." Opposing mainly what's not on the agenda and doing nothing to oppose what's actually being done is a response that makes no political sense. Why do leftists insist on that? That has a lot to do with their own perception of the character of Iran's Islamic government, as well as explicit or implicit abandonment of the anti-imperialist framework of thinking that came from Marxist and other traditions on the Left and explicit or implicit adoption of liberal progressivism and internationalism.
More generally, both the actual character and others' perceptions of governments, political parties, and social movements at odds with US hegemony have everything to do with the outcome of a politico-economic possibility that is on the horizon now due to many objective and subjective reasons (the subprime US economy, the economic rise of China, the politico-economic recovery of Russia, the consolidation of the euro, relatively high oil prices, the Bolivarian Revolution in Venezuela and its various allies in Latin America, the rising opposition to the US client state and its corporatist union bureaucrats in Egypt, the weakening of the Kemalist military hegemony in Turkey, multiple challenges to Musharraf in Pakistan, belated but not insignificant Indian Communist objection to India's consent to US hegemony, and so on): the possibility of checking US hegemony.
If Iran were actually what both the US power elite and many leftists claim it is, i.e., a country that is run by a government that has already lost, or is soon to lose, legitimacy in the eye of its populace, its resistance to the empire would be doomed, no matter what anyone says. It would be a lost cause, and I would not bother trying to educate anyone about it, for _a country whose own people do not defend cannot be defended_. But that is not the case here. Therefore, we shouldn't help the US power elite talk themselves into believing their own dangerous propaganda.
The actual character of Iran's government, unlike that of Saddam Hussein's Ba'ath Party dictatorship for instance, gives leftists a lot to work with. So does the character of Hamas and Hizballah, Iranian support of which is a major bone of contention. This matters less in the USA, the UK, and Canada (where leftists are weak) and Japan (where leftists, due to historical reasons, generally do not subscribe to liberal internationalism) than in continental Europe, where leftists are not without power and on whose governments Washington, together with Sarko the American and his Socialist Foreign Minister, is putting much pressures to economically isolate Iran. -- Yoshie