Somehow there's a theory that's been floating around, mostly in conservative circles but I'm seeing it from some number of liberals as well, that Iran is somehow closely linked to either ISIS or al-Qaeda or both.
Now I could just file this under one of two things: Americans' natural aversion to Iran and apparently Muslims in general acting a lot like our general aversion to aboriginal peoples in the 19th century in a clumsy set of "understandings" of the nature of a strange culture and people. Or Americans' general lack of knowledge of foreign relations and our clumsy way of clumping together things that don't actually have anything to do with one another because we don't possess any actual facts.
But since the idea keeps popping up, and conservatives seem to feel there are documents in the bin Laden stash that was just released that would prove it (that were not released, as part of some conspiracy to get us to go along with a treaty negotiated with Iran on their nuclear programme), I feel obliged to point out that it's absurd and dumb and points out one of the reasons why many of our interventions in that region have been counterproductive: namely, that we don't know what the sides even are that we are intervening for. I could reject it without argument because it presents no evidence but it also ignores and elides several sets of evidence on the ground that contradict these theories.
1) ISIS subscribes to a particular strain of Islam, largely drawn from Sunni traditions (and mixing in a lot of crazy, as religious zealots are wont to do). al-Qaeda, particularly the Iraqi version around which ISIS is partly formed, does likewise. But they're not all that fond of each other, at least not any more. ISIS may have started out as a branch of al-Qaeda but they're mostly engaged in warfare against other Muslims for territorial dominance. Which wasn't the primary interest of al-Qaeda, which is mostly concerned with interference from Western powers in Muslim culture (and then also imposing a variety of Muslim culture with a degree of crazy of its own concoction once that influence and interference is or was removed).
al-Qaeda under al-Zawahiri might be mostly engaged in a pissing match with al-Baghdadi for control over radical or fundamentalist Islamic groups in the region, but that they are not all that fond of each other does seem to be true, and does seem to be a factor cutting against the idea that these groups will necessarily fall under the same rubric and same features of being a threat. Our goal should not be to lump them together under a lazy and mildly bigoted narrative arc but take stock of the divisions in our rivals and use them when needed to strategic advantage.
2) They're both not all that fond of Iran, which is the primary source of Shi'a Islam. The idea that they would cooperate with Iran is about as absurd as the idea that they would have cooperated or been operating with the support of Saddam government in Iraq. This argument smells mostly of the narrative supports being used to sell us on a potential war with Iran (in the same way this was used to sell us on a war with Iraq) rather than any rational explanation of the facts on the ground. This is to say that it mostly smells like bullshit. It doesn't need to be categorically rejected, but it requires a stronger burden of proof having already been an argument made and been shown as demonstrably false before.
3) The main area of territory controlled by ISIS is in Syria and Iraq. Both of these were effectively client states of Iran prior to the civil war in Syria and the emergence of ISIS in Iraq (the Kurdish area of Iraq might be an exception here, but particularly the eastern and southern parts of Iraq are Shi'a country, effectively). Iran's main interest would be the stability and maintenance of these regional partnerships. ISIS is exactly antithetical to that. Which is why Iran's been backing a variety of Shi'a militia groups in Iraq to go fight ISIS.
4) al-Qaeda, particularly the Iraqi version, has spent most of its time killing other Muslims, mostly Shi'a Muslims. They're not going to be all that concerned with Iran either. bin Laden might be a different case than Zarqawi. Which wouldn't be all that surprising that separate cells of a terrorism organisation would have and form different goals. Zarqawi's dead though. So is bin Laden. So what they thought or tried to do is not as relevant to how their respective groups have behaved since under new leadership.
5) Regardless of these facts, it seems less than clear that it is established that either ISIS or Iran is a substantive threat to American interests or national security. We have allies in the region that aren't all that happy and most of them are doing things about it. But whether that imposes a danger to Americans or the US nation-state is not established by other countries having problems.