At what point does criminal activity threaten national security? When can a nation respond to it as to an enemy? These are the questions Jack Ryan must consider when he hears the awful news: Colombian drug lords, tired of being harassed by US law enforcement agents, have assassinated three high American officials, and that is just the beginning. The message is clear: Leave us alone.But they have pushed too far. Someone steps quietly over the line they have drawn, deploying covert-action teams in Colombia. At home, too, men armed with the most sophisticated tools their country can devise begin to take the fight to the enemy. But does anyone know who the real enemy is, and how much action is too much? Who is doing what? Which line has been crossed? Ryan and his ''dark side'', a shadowy field officer known only as Mr. Clark, must find out. They expect danger from without ¿ but the danger from within may be the greatest of all.