Overall I found the Steyn piece reasonably accurate, given that Mr. Steyn seemed to stop his historical analysis at the eve of the revolution. Steyn makes the same singular mistake I've seen in every mainstream analysis on Iran: that Iranian history begins in 1979. I do disagree with current Iranian national security policy, but by knowing the full context I understand where Iran is coming from. With that, I picked out some key words from the Steyn article to formulate my argument: the buffer zone, uniting Islam, military credibility, and democracy.
The first key phrase, the buffer zone, arguably defines Iran's foreign policy. If you look past the revolution, even past the Muslim conquest of Persia in 650 AD, you'll find that not only have Iranians reaped the economic benefits of their central location, but Iran has also felt the pain of numerous outside powers carving out its territory. By 1979 the US was just another in a long line of outside actors laying claim to Iran. Indeed, thanks to the foreign pressure Iran formed a cultural solidarity unlike her neighbors in Iraq and Afghanistan (despite the latter's own experiences with empire-minded foreign powers).
Unlike the tribal nature of Afghanistan or even Saudi Arabia, Iranians identify themselves as Iranians first, then Baluchis, Azeris, etc. Combine this sense of nationalism with Iran's waxing and waning periods of power and you can begin to grasp why the current regime preaches such an aggressive security policy. Persia used to be a force to be reckoned with, before the Muslim conquest and after, when Persia notably contributed arts and letters to Muslim culture. (For example, as French was adopted as the language of the courts in Medieval Europe, so was Persian made the court language during the Islamic golden years of c.650 AD to the mid-18th century). Iranians are tired of outside powers dictating how they should run their affairs; Iranians want to dictate how they run their affairs.