obstacles to preventing or stopping terrorism in the modern world?

Terrorism is the calculated unlawful use of physical force or threats of violence against persons or property in order to intimidate or coerce a government, organization, or individual for the purpose of gaining some political, economic, religious or social goals. Terrorism is a potential threat to societal stability in our and other high-developed countries.

In your opinion, what are the major obstacles to preventing or stopping terrorism in the modern world?