In March 1968 he participated in a meeting at Halle held to discuss the government's continuing campaign of persecution against Robert Havemann who at that time was regarded as a dissident, being held under house arrest following the loss of his Party Membership and university post. At the meeting Eisenfeld openly criticised the East German draft constitution which the government had published as part of a move to align East Germany's constitution more closely with the realities of its power structure. He also expressed his support for free expression and for the Communist Reform Movement becoming mainstream in Czechoslovakia, under the leadership of Alexander Dubček. East Germany's state security establishment responded to Eisenfeld's speech by launching what they termed "Operation Economist" against him, making plans to arrest the dissident together with his two brothers, Ulrich and Peter. Before they could do this, however, in May 1968 the three brothers traveled to Prague, where they stayed for three days, and which they apparently managed to accomplish without the Ministry for State Security learning of their travel plans till it was too late to stop the brief trip. He flooded the "readers' letters" pages of East German newspapers with letters (which were never published), fulsome in support of the "Prague Spring". Following the Warsaw Pact invasion of Czechoslovakia on 20/21 August 1968, on 23 August 1968 Eisenfeld sent a telegram of solidarity to the Czechoslovak Embassy, containing the exhortations to "Stand firm and not give up hope" ("Halten Sie stand – Behalten Sie Hoffnung").