Jundallah is basically a formal way of saying 'Al Qaeda- Iran Front'.
Al Qaeda is and always has been a natural enemy of Iran.
How can Iran not be fighting the war on terror if they are always fighting terrorist, whether they be the American/Israeli backed PJAK or Al Qaeda backed/inspired Sunni offshoots like Jundallah.
At any rate, Iran and the US both have incentives to go after al Qaeda together.
Al Qaeda kills Americans and Iranians just the same.
