When does attacking hard targets have a higher impact than attacking soft targets?

When does attacking hard targets have a higher impact than attacking soft targets?

What are the methods of terrorists? (e.g., targeting civilian populations)

Why do terrorists use the tactics of terrorism?

Which terrorist groups were active in previous generations? Did they achieve their goals?

What are some of the major terrorist groups active today?
1. Which are the most serious concern for the United States?
2. Most serious threat worldwide?