Iran is stuck between a rock and a hard place on ISIS and Iraq. Taking responsibility for security in Iraq – or even significantly contributing to it – would be a huge undertaking. But a fragmented Iraq on its border is a first-order concern for Tehran - it can’t just sit by with fingers crossed. The choice is complicated by Syria. Iran can’t continue to pursue its interests in Syria at the same level if it is mired in Iraq as well. It is likely that Tehran will have to choose, and it will choose Iraq.