Martin S. Alexander 
Knowing Your Friends [PDF ebook] 
Intelligence Inside Alliances and Coalitions from 1914 to the Cold War

Little attention has been paid to the murky, ultra-business of gathering intelligence among and forming estimates about friendly powers, and friendly or allied military forces. How rarely have scholars troubled to discover when states entered 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 game in town. The twentieth century has been chosen to enhance the coherence of and connections between, the subject matter of this under-explored part of intelligence studies.