Term employed to mean different things at different times by various groups in the history of C20 architecture, but mostly applied to mean the architectural principles behind the International Modern Movement led by such personalities as Gropius and Mies van der Rohe subscribing to the so-called Machine Aesthetic and to Functionalism. However, the word has been so loosely used that some expanded explanations are necessary.
Classical and Renaissance architectural treatises argued that architecture was a science with principles that could be understood on a rational basis. C18 and C19 theorists, notably J. -N. -L. Durand, Viollet-le-Duc, Semper, and others also argued for reasonable approaches to design derived from the culture of the European Enlightenment. Those arguing for C20 Rationalism did not have any one coherent theory, but made assumptions that architectural and urban problems could be solved primarily through an abandonment of Historicism and of movements such as the Arts-and-Crafts, Art Nouveau, and Expressionism (which they regarded as dead-ends), thus creating a tabula rasa on which to start again. They tended to be messianic in their desire for a new world, better architecture, Socialist structures, and a belief in the inherent rightness of what they were seeking, drawing on a Machine Aesthetic to achieve an appropriate image.
Advocates of Rationalism evolved certain principles by which their aims were to be met. First, architecture, industrial design, and planning could be used for social engineering and educational purposes, and so design had a moral meaning (a notion drawn partly from the writings of A. W. N. Pugin and Ruskin). Second, strict economy, cheap industrialized building methods, and a total absence of ornament were to be employed to achieve a minimum standard for everyone's habitation. Third, prefabrication, industrial technologies, and mass-production at all levels were to be used in the making of the new environment, but, even if traditional methods of construction were employed (bricks, after all, are mass-produced, standardized, prefabricated building-components), buildings should look machine-made in their pristine state (so brickwork was disguised by being covered with smooth render). Fourth, wholesale clearances, demolitions, and the destruction of existing urban fabric were deemed to be essential so that vast housing-estates could be erected. Lastly, form itself should be evolved for constructional, economic, functional, political, and social reasons, and so was not (in theory) subject to individual fancy (but in fact was largely determined by a few paradigms).
In practice, Rationalism encouraged an approved International style from which all historical and decorative elements were expunged, drawing on influences from e.g. Constructivism and de Stijl. Among key buildings were Gropius's Bauhaus, Dessau (1925–6), Le Corbusier's Maison Stein, Garches (1927), and houses at the Weissenhofsiedlung, Stuttgart (1927), while theoretical and unifying bases were provided by CIAM and certain writers, notably Giedion and Pevsner.
It is one of the curiosities of Rationalism that it flourished in Italy under Benito Mussolini's Fascist regime (1922–43), and in fact International Modernism was also called Rationalism by Gruppo 7. Terragni was perhaps the most distinguished Italian Rationalist, with his Fascist Party Headquarters, Como (1932–6). Gruppo 7 expanded to form the Movimento Italiano per l'Architettura Razionale (MIAR), inspired partly by Futurism. After the 1939–45 war Rationalism was adopted, virtually as the de rigueur style of Western Europe and America. Looked at objectively, it was just another style, drawing its motifs from a limited range of features approved in the 1920s, and owing very little to rationalism at all, but more to the desire for images thought to be appropriate for the times, and that, in any case, were usually only metaphors of mass-production, modernity, and industrialization.