But I thought the Russians were on the side of the Serbs this time. Why would Soviet agents want to hurt the Serbs?
Even pre-communist Russia supported the Serbian nationalists in WWI, I believe.
The short version, because my copy of the book is at home:
During the war, the Soviet moles at State had us backing, both financially and with materiel, the Communist partisans in Yugoslavia rather than the anti-Communist ones, who we cut off at the pockets. Both sets of partisans were Serbs.