A number of key military strategists have stressed the central importance of intelligence to success in conflict. Intelligence can be seen as a critical tool of power for states in conventional conflict scenarios. The interaction between terrorist and state actors has often been analysed using Game Theory approaches, since these allow for a notion of the critical interdependence between each side of the equation. Terrorists will always be aiming to outwit the state, and they themselves will need information about the capabilities of counterterrorist policies and agencies in order to do so. The gathering of intelligence by a state on a terrorist group can be seen as a legitimate and effective element of proactive and pre-emptive counterterrorism policy: essentially, the "foreknowledge" of which Sun Tzu spoke. It should be noted that, particularly in the post-9/11 era of terrorism and counterterrorism, not all significant intelligence on terrorism is gathered by state intelligence agencies.