The message is clear in attacks like this. The message is that we (the attackers) will persist until you Americans give up in disgust with the slaughter, the mindless (to you) inter-communal violence, the loss of life and limb on your side, the vast expenditures that could be spent on things like health insurance for the poor in the USA. The specific message is variously that:
We Sunni Arabs will fight before, during and after your withdrawal to end up with at least a parity of power with the Kurds and Shia Arabs. Our brothers "outside" will help us. They will tell you Americans whatever you are gullible enough to believe about their relationship to us and to the the Israelis, but they will help us. When you are gone we will fight the Shia, the Iranians, maybe the jihadis if they try to rule us, but we will fight.
The message from the Shia Arabs is that we will persist in our efforts to live in dignity and sovereignty in Iraq. We, too, have friends.
The message from the jihadis is that "God Wills It," and we will persist in His path until all submit. Death is nothing.
The message from all: Who are you Americans to tell us that our struggles are "incomprehensible?" Who are you? pl