Sun Tzu said that it is as important to know yourself - and by extension, your allies - as it is to know your enemies. This is a study of the murky, ultra-sensitive business of gathering intelligence among, and forming estimates about, friendly powers, and friendly or allied military forces. States sometimes enter into coalitions or alliances mainly and explicitly because their intelligence evaluation of the potential partner concluded that making the alliance was, from the originator's national security interest, the best thing to do. This collection investigates this aspect of intelligence.