Several hundred thousand members of the Red Army were stationed in East Germany when that state was reunited with its western counterpart. The peaceful transfer of these soldiers to their homeland produced a welcome outcome to a potentially explosive situation. Through an investigation of the strategies of German and Russian decision-makers, Celeste A. Wallander explores what conditions facilitate or hinder international cooperation in security matters.Wallander spent the months and years after the fall of the Berlin Wall interviewing officials and politicians from Germany and Russia. She reveals how these individuals assessed and responded to potential flashpoints: the withdrawal of Russian military forces from Germany, the implementation of arms control treaties, the management of ethnic and regional conflicts. She also examines the two states' views on the enlargement of NATO.The first detailed account from both countries' perspectives of the extraordinary contraction of Russian power and the implications of German unification, Mortal Friends, Best Enemies clearly depicts the important role European and global institutions played making the military disengagement possible. Wallander draws on these findings to develop a new institutional theory of security relations. In it she defines the techniques that international institutions can use to help states solve obstacles to security.