Warschauer Vertrag
The Social-liberal coalition of SPD and FDP had lost its majority after several Bundestag MPs (like former FDP ministers Erich Mende and Heinz Starke or SPD partisan Herbert Hupka) had left their party and become members of the CDU/CSU opposition to protest against Chancellor Willy Brandt's "Neue Ostpolitik", especially against the "de facto" recognition of the Oder-Neisse line by the 1970 Treaty of Warsaw.