The difficult Brexit negotiations are a clear demonstration of why success in negotiations heavily depends on having a clear mandate. Theresa May made the mistake of first negotiating a result externally with the EU and then hoping to get an internal mandate for it – a wrong order with considerable consequences. Now Europe is plunged into a crisis and May almost lost her own office. Within the EU the Brexit sub-dealers ask themselves: can we still agree on results with such a negotiating partner if it is not clear what weight her word has within her own ranks? How reliable will a next agreement be? And is it at all still possible to rely on her statements?