What is the theme in Night Soldiers by Alan Furst?

Brotherhood is a theme in this book. The Soviet Union recruited their espionage agents from many countries, as shown by the nationalities of the men who share Khristo's training class. It may well have been the intention of the Soviets to choose these men in hopes they would not find a commonality, but in the case of Khristo, Kulic, Voluta and Goldman, the experience of becoming a unit and working together formed a brotherhood that could not be broken.