Clear and Present Danger, Tom Clancy

Colombian drug lords, bored with Uncle Sam's hectoring, assassinate the head of the FBI. The message is clear: Bug off!
At what point do these druggies threaten national security? When can a nation act against its enemies? These are questions Jack Ryan must answer because someone has quietly stepped over the line.
Does anyone know who the real enemy is? How much action is too much? Which lines have been crossed? Ryan and his "dark side", a shadowy field officer known only as Mr. Clark, are charged with finding out. They expect danger from without... but the danger from within may be the greatest of all.