After World War I, Mies began, while still designing traditional neoclassical homes, a parallel experimental effort. He joined his avant-garde peers in the long-running search for a new style that would be suitable for the modern industrial age. The weak points of traditional styles had been under attack by progressive theorists since the mid-nineteenth century, primarily for the contradictions of hiding modern construction technology with an facade of ornamented traditional styles. The mounting criticism of the historical styles gained substantial cultural credibility after World War I, a disaster widely seen as a failure of the old world order of imperial leadership of Europe. The aristocratic classical revival styles were particularly reviled by many as the architectural symbol of a now-discredited and outmoded social system. Progressive thinkers called for a completely new architectural design process guided by rational problem-solving and an exterior expression of modern materials and structure rather than the superficial application of classical facades.
While continuing his traditional neoclassical design practice Mies began to develop visionary projects that, though mostly unbuilt, rocketed him to fame as an architect capable of giving form that was in harmony with the spirit of the emerging modern society. Boldly abandoning ornament altogether, Mies made a dramatic modernist debut with his stunning competition proposal for the faceted all-glass Friedrichstraße skyscraper in 1921, followed by a taller curved version in 1922 named the Glass Skyscraper.
He continued with a series of pioneering projects, culminating in his two European masterworks: the temporary German Pavilion for the Barcelona exposition (often called the Barcelona Pavilion) in 1929 (a 1986 reconstruction is now built on the original site) and the elegant Villa Tugendhat in Brno, Czech Republic, completed in 1930.
He joined the German avant-garde, working with the progressive design magazine G which started in July 1923. He developed prominence as architectural director of the Werkbund, organizing the influential Weissenhof Estate prototype modernist housing exhibition. He was also one of the founders of the architectural association Der Ring. He joined the avant-garde Bauhaus design school as their director of architecture, adopting and developing their functionalist application of simple geometric forms in the design of useful objects.
Like many other avant-garde architects of the day, Mies based his own architectural theories and principles on his own personal re-combination of ideas developed by many other thinkers and designers who had pondered the flaws of the traditional design styles.
Mies' modernist thinking was influenced by many of the design and art movements of the day. He selectively adopted theoretical ideas such as the aesthetic credos of Russian Constructivism with their ideology of "efficient" sculptural constructions using modern industrial materials. Mies found appeal in the use of simple rectilinear and planar forms, clean lines, pure use of color, and the extension of space around and beyond interior walls expounded by the Dutch De Stijl group. In particular, the layering of functional sub-spaces within an overall space and the distinct articulation of parts as expressed by Gerrit Rietveld appealed to Mies.
The bold work of American architects was greatly admired by European architects. Like other architects who viewed the Wasmuth Portfolio and its associated exhibit, Mies was enthralled with the free-flowing spaces of inter-connected rooms which encompass their outdoor surroundings as demonstrated by the open floor plans of the American Prairie Style work of Frank Lloyd Wright. American engineering structures were also held up to be exemplary of the beauty possible in functional construction.