This is has been the case with most attacks in Iraq. And the U.S. response is always the same: accuse Iran of interfering in Iraq.
Stop accusing Iran and start punishing Iran.
The day is coming slowly, just as it did for Iraq, but it will get here.
These things take time. That being said at some point soon (in the next couple of years) we will have to deal with Iran. If we are successful in Iraq that will deal a huge blow to the Mullahs.