[ by Charles Cameron — sometimes a flurry of tweets can be the thing itself — see also today Michael Kenney on al-Muhajiroun ]
Assuming that ISIS was behind the Tehran attack, as it has claimed, I have a few observations.
1) Attacking Iran differentiates ISIS from al-Qaeda. AQ has probably wouldn’t attack Iran due to the contours of the AQ/Iran relationship. Similarly, ISIS propagandized against the Taliban by arguing that they wouldn’t fight Pakistan, while ISIS was killing Pakistani soldiers. So there’s an obvious propaganda play ISIS can make here for a Sunni audience, parallel to its earlier Pakistan propaganda.
2) Iran is at the forefront of pushing ISIS’s caliphate back. So this is similar to other ISIS attacks against coalition countries. Nor is it clear that ISIS is concerned about provoking Iran: They already have provoked Iran. It’s fighting them on the ground now.
3) Wild card possibility: Unlikely, but could ISIS be making a play for Sunni state support, similar to what al-Qaeda gets? There are various reasons that I would bet against this, but I would put it on the table as a possibility, however remote.