Realistically and scientifically speaking, a dirty bomb, aside from the initial explosion, is overall relatively harmless. According to the United States Nuclear Regulatory Commission (NRC), “the conventional explosive would be more harmful to individuals than the radioactive material”.

In reality, the threat of a dirty bomb is yet another illusion. Its aim is to spread radioactive material through a conventional explosion. But almost all studies of such a possible weapon have concluded that the radiation spread in this way would not kill anybody because the radioactive material would be so dispersed and, providing the area was cleaned promptly, the long-term effects would be negligible.

So the only real danger a dirty bomb poses is fear and anxiety being spread about it's (nonexistent) threat. That is the only factor that makes it effectual for a potential terrorist. It would seem that sound opposition to terrorism would be widespread education of the facts on matters such as these. Yet the media who are supposedly against terrorism give power to such groups by reinforcing the lies that cause concern amongst the victims.

So that being said, if someone really "planted a dirty bomb in a major US city" the real source of terror would be the propaganda put out by our own government.

The facts are that Islamic terrorism had no foothold whatsoever until the influence of foreign interventionism. The supposed agendas to counter it have reinforced the potential for such organizations to grow from weak coalitions bound to collapse to "ISIS". Obviously the rhetoric to promote the destruction of terrorist activities is truly an effort to gain public support for actions that actually feed its relevancy. One could argue it has been merely the unforeseen consequences of humanitarian goals, but in truth it seems to be more like an imperialistic effort to destabilize other nations for the interests of governments who have no business being involved.