Cette thèse propose une analyse des notions de justice sociale et d'État-providence au prisme de la politique sociale du SPD dans les années 1980. À la fois parti d'opposition au Bundestag à Bonn et parti au pouvoir dans certains Länder en RFA, le SPD se trouvait dans une position ambiguë, propre au fédéralisme allemand. Afin de peser sur les réformes sociales initiées par le ministre du Travail Norbert Blüm (CDU) pour résoudre la crise de l'État-providence, les sociaux-démocrates adoptèrent une stratégie alternant opposition et coopération avec le gouvernement Kohl. Dans le même temps, ils engagèrent un travail de refondation programmatique en vue de reconquérir le pouvoir en 1990. Aux divisions générationnelles communément admises par la recherche se substitua une fracture entre une « aile sociale » adepte d'une politique traditionnelle et une « aile réformatrice » sensible aux valeurs post-matérialistes et « écosocialistes ». Du fait de son immédiateté, le processus d'unification de l'Allemagne (1989-1990) constitua un révélateur permettant de juger, à l'épreuve des faits, la validité des programmes et des discours sociaux-démocrates. Malgré les propositions concrètes de Rudolf Dreßler pour améliorer l'union sociale entre la RFA et la RDA, le SPD ne parvint ni à se faire entendre sur la question de l'unité allemande ni à imposer sa volonté de refonder l'État-providence. Les réserves d'Oskar Lafontaine sur l'emballement des coûts économiques et sociaux contribuèrent à entretenir la confusion concernant la position du SPD sur l'unité allemande. / This doctoral thesis analyses the concepts of social justice and the welfare state in light of the social policies of the Social Democratic Party of Germany (SPD) in the 1980s. Both in the opposition in the Bundestag in Bonn and in office in some West German Länders, the SPD found itself in an ambiguous position, peculiar to German federalism. The Social Democrats took on a strategy based alternatively on opposition and cooperation with Helmut Kohl’s government, in order to influence the welfare reforms introduced by the Labour Minister Norbert Blüm (Christian Democratic Union, CDU). The SPD simultaneously started to overhaul its political platform with a view to taking back power in 1990. The generational conflicts, which have been widely established by researchers, gave way to a new divide between a “social wing” advocating a traditional policy and a “reforming wing” drawn towards post-materialist and “eco-socialist” values. By virtue of its immediacy, the German unification process (1989-1990) proved to be a litmus test, which allowed the efficiency of the SDP’s agenda and rhetoric to be evaluated. In spite of Rudolf Dreßler’s concrete propositions to improve the social union between East and West Germany, the SDP failed both to share its views on German unification and to impose its determination to overhaul the welfare state. Oskar Lafontaine’s reservations about the economic and social costs spiralling out of control contributed to a blurring of the lines on the SDP’s position on German unification.This doctoral thesis analyses the concepts of social justice and the welfare state in light of the social policies of the Social Democratic Party of Germany (SPD) in the 1980s. Both in the opposition in the Bundestag in Bonn and in office in some West German Länders, the SPD found itself in an ambiguous position, peculiar to German federalism. The Social Democrats took on a strategy based alternatively on opposition and cooperation with Helmut Kohl’s government, in order to influence the welfare reforms introduced by the Labour Minister Norbert Blüm (Christian Democratic Union, CDU). The SPD simultaneously started to overhaul its political platform with a view to taking back power in 1990. The generational conflicts, which have been widely established by researchers, gave way to a new divide between a “social wing” advocating a traditional policy and a “reforming wing” drawn towards post-materialist and “eco-socialist” values. By virtue of its immediacy, the German unification process (1989-1990) proved to be a litmus test, which allowed the efficiency of the SDP’s agenda and rhetoric to be evaluated. In spite of Rudolf Dreßler’s concrete propositions to improve the social union between East and West Germany, the SDP failed both to share its views on German unification and to impose its determination to overhaul the welfare state. Oskar Lafontaine’s reservations about the economic and social costs spiralling out of control contributed to a blurring of the lines on the SDP’s position on German unification.