1. dirty bomb kills 3 people, injures 40, leaves radioactive contamination in a section of the Washington, DC, subway
2. two things to do in response:
(1) post guards at all subway entrances in DC area and set up inspection queues similar to airport checkpoints, causing slowdowns and inconveniences for all
(2) let's say the perpetrators were known to have been from Syria ... then make threatening gestures toward Syria, maybe strike some targets there
3. lots of speeches decrying the evil terrorists and what they did and swearing that "we will hunt them down and bring them to justice"
If the goals of the terrorists include (1) getting attention, and (2) interrupting business as usual in the US, then they conclude this is a successful outcome.