"So what are we going to do about it?"
What we should do, is close the border with Iran. No trade, worshippers, nada.
Then evacuate an area about 5 clicks wide along the border and declare it a free fire zone. Then slaughter anything that moves in that zone.
>What we should do, is close the border with Iran
Ha. This is a joke right? We can't even close the borders in our own country, and we should magically do this in Iraq...against people who are actually armed and dangerous. Yeah, we're already strained for resources, this is not going to happen. So we expect Iran to wall up their borders, when we can't even do the same at home.
Look, the big picture is that there's a geopolitical chessboard and every side sees how it gain advantage in the region. The US has its "militants" in the region despite the fact that the majority of Sunni's and Shi'ites both want us gone. So we are an unwelcome presence. Iran is allowing its militants to defend Shi'ites in the region as well. Every country is pursuing its interests in Iraq. In some ways, the Iranian militants may be more welcome to the largely Sh'ite majority in Iraq than the American militants are.
So we can complain all day about what Iran is doing, that they are a foreign presence meddling in Iraq's affairs, but that is precisely how the Iraqis and the rest of the world sees us.