From The Age of the Unthinkable by Joshua Cooper:
In 1978 the American political scientist Robert Jervis […] was interested in a specific problem of nation-states that he called the “security dilemma”. The dilemma went something like this: every state wants to feel secure, but it is doomed in this quest because the very steps it takes to feel more secure almost always make other states feel less secure. The result is a sort of accelerating uneasiness, a trap that ultimately makes everyone less safe. […] Is it now more expensive to attack or to defend? The real math of our present moment yields the opposite answer from what security optimists postulated in the 1990s: attacking is cheap, and, if Jervis is right, it suggests that we’ll see more violence in the future rather than less. The 9/11 hijackers spent less that $1 million [US dollars] to attack the United States. The cost to try to prevent a similar attack —in police, airport security, and other systems— runs to a million dollars an hour in the United States alone.