Everything changed in January 1933 when the Nazis took power and lost no time in transforming Germany into a one-party dictatorship. Although Communist activists were at the top of the government hit list, others with a record of activism in the SPD and the union movement also attracted the attention of the authorities. Lehmann was deprived of his offices and almost at once arrested, spending time in government detention during March/April 1933. In June 1935 he was again taken into "investigative custody", identified as the leader of a resistance group in Berlin comprising members of illegal organisations: Social Democrats and Trades Unionists. A few years later the assassination plot against Hitler failed in its primary objective but succeeded in making the government acutely nervous. They dusted down a pre-prepared plan and on the night of 22/23 August 1944 mass arrests took place across Germany in an exercise identified as "Aktion Gitter". What the people arrested had in common was that before 1933 they had been listed as active politically, usually as members of the Communist Party, the SPD or the trades union movement. Many of those sought, especially Communists, had long since fled abroad. Others had died, sometimes simply of old age. But that still left more than 4,000 who were arrested: Hans Lehmann was one of these. In January 1945 the special People's Court sentenced him to a year in prison for "Backing the enemy" ("Feindbegünstigung").