Though having roots in various forms in Ancient Rome, France during the Enlightenment Rationalism and the Soviet Union in the early 20th century (ASNOVA being the most recent group of artists calling themselves "The Rationalists) an Italian branch developed in between the 1920s and 1940s led by the Gruppo 7. For Canadians, the name should be familiar as it translates into the Group of Seven. Their objective was to strike a balance between the Futurism and Classicism. Which couldn't have been too hard given the high octane level of impractical testosterone and vulgarity (one could add irrational) found in the Futurist movement.
It's important to note that rationalism was further pondered and incorporated into the works of French architects (like Henri Labrouste who lived in Rome for six years) as well leading to what was known as modernism (avant-garde). There's quite a bit of overlapping of influences between French and Italian designers during this period as you can see.
Rationalism emphasized less the science of architecture - focusing on only the functionality of a city for example - and more on the environment necessary to be incorporated into any construct. That is, history and the physical environment, for instance, should be considered in the conceptualization of an architectural design.
In short, the solve any real world problems took reason and, well, rationalism.
This was especially true during the second wave of Rationalism in the 1960s called Neo-rationalism led mostly by a group called Tendenza (Tendency).
One such artist was Aldo Rossi who designed the Quartier Schutzenstrasse in Berlin, Germany (pictured).
