As far as the US role is concerned, it is an Obama initiative. He is negotiating with Iran under "Sole Executive Agreement" power which bypasses Congress.
Presidents have sought ways to bypass congress since Washington. When people like what the president is doing they justify and excuse it, if not they oppose it. We all know that the sole reason congress wants to weigh in on this is to kill it.
Killing one person is murder, killing 100,000 is foreign policy