He escaped and fled to Leipzig where he started a career as a journalist and later editor for social democratic and socialist papers.

In a speech given to the 1916 SDP conference, he remembered the 'August enthusiasm': He became famous, during World War I, as a member of the Lensch-Cunow-Haenisch group, a nationalist tendency within SPD which based the support of the SPD for the "war credits" (funding for the German military effort) in the Reichstag on a Marxist theory suggesting that a German victory in World War I could be used by SPD, which was still a dominant force in European socialism, to transform Germany into a socialist state and to trigger socialist revolutions in the defeated countries.

His associates in this movement were Heinrich Cunow and Paul Lensch, both former left-wing social democrats and Marxists close to Rosa Luxemburg.

[3] When it became evident Germany would lose the war, Haenisch became part of the reformist stream led by later President Friedrich Ebert.

Haenisch realized the increasing threat to German Parliamentary Democracy emerging from totalitarian communism and fascism, and became one of the founders of "Reichsbanner", a paramilitary organisation founded to protect the Weimar Republic and rallies of democratic parties like SPD, German Democratic Party and Zentrum.