If we had really wanted to destroy ISIS we should have supported Assad, who might have been able to destroy them with our help. Of course, then you end up keeping Assad in power in Syria. But you can’t have it both ways. In Syria you have to choose between Assad and ISIS; choosing the “good” rebels is a recipe for defeat of the good rebels and their American patrons.
In part, the ISIS move into Iraq may be prompted by Assad’s success against them in Syria. ISIS was losing in Syria, and decided maybe it would have better luck in Iraq. If it had been more successful in Syria, it might have stayed there and taken over the Syrian government. Creating a greater Sunni Iraq-Syria is a fallback position. In Syria ISIS did okay in the Sunni areas, but the Assad government was Alawite allied with the Shiites, making it difficult for ISIS to take over important parts of the country, like Damascus. They have been so successful in Iraq because they are taking over Sunni areas of the country, where they are to some extent welcomed, more so than America was after its invasion of Iraq. However, ISIS will probably run into the same problem in Sunni areas of Iraq that it ran into in Syria, making it questionable whether they can take over Baghdad, for example, just as they could not take over Damascus. As a result they will try to create a Sunni regime of some kind in the conjoined Sunni regions of Syria and Iraq, in the process probably freeing the Iraqi Kurds from Baghdad’s rule.
In any case, the solution to Iraq’s problems does not lie in Syria. I also do not favor an alliance with Iran against the ISIS Sunnis. The best thing we have done so far for Iran is kill Saddam Hussein. Helping them destroy the Sunni insurgency would rank right up there with our earlier mistake. If two of our enemies are fighting each other, the Sunni terrorists and the Iranian terrorists, let them kill each other; don’t stop them. If one side ends up about to win a great victory, then we may want to intervene to maintain some kind of balance of power, but we are not there yet.
I want to blame George W. Bush and Republicans for this mess, because of their invasion of Iraq that had nothing to do with 9/11 or weapons of mass destruction. However, it looks like the Middle East was a powder keg about to go off. All of these religious and ethnic tensions, not to mention the Israeli-Arab conflict, were already hot, but it was stupid for Bush and company to light the fuse that set off the powder keg. If it had gone off in some other way, we might have been better able to manage it, or maybe not. But as it is, virtually every country in the Middle East is in turmoil or on edge. Although everyone seems to prefer to use the ISIS acronym (Islamic State in Iraq and Syria), the other acronym, ISIL (Islamic State in Iraq and the Levant), more accurately shows the intentions of the terrorist group. Their ambitions extend overthrowing to Lebanon, Jordan, and Israel. You can add those countries to the existing list of Egypt, Tunisia, Libya, and others that have already been destabilized. And ISIS-ISIL has plenty of support, financial and otherwise, from the Sunni petro-states, starting with Saudi Arabia.