When Rau took charge of the XI Brigade, he might have been at odds with his predecessor, Richard Staimer, the future son-in-law of KPD leader Wilhelm Pieck. This was the time of the Great Purge which had its echos in Spain, and it could be perilous to have powerful enemies. André Marty, the chief commissar of the International Brigades based at Albacete, was also an executor of the Great Purge in Spain. Following Rau's injury, Marty managed to imprison him under a pretext for a brief time. A report, written in Moscow in 1940, described Rau as a "political criminal", who had had contact with the Spanish anarchists and members of the Workers' Party of Marxist Unification (POUM), which was demonized as "Trotskyist". These were serious allegations in this time, when accusations of Trotskyism frequently led to a death sentence if the accused was within reach of the authorities.