From Hitchcock and Johnson, The International Style: Architecture Since 1922 (pp.28)

When did this new period emerge? Early 1920s
¥ÒThere are certain times when a new period truly begins despite all the preparation that may be traced behind the event.  Such a time came immediately after the War (ie. WWI), when the international style came into being in France, in Holland, and in Germany.  Indeed, if we follow the projects of the War years made by the Austrian Loos and the Italian SantÕ Elia, it may appear that the new style was preparing on an even broader frontÉ
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¥It is particularly in the early work of three men, Walter Gropius in Germany, Oud in Holland, and Le Corbusier in France, that the various steps in the inception of the new style must be sought.  These three with Mies van der Rohe in Germany remain the great leaders of modern architecture.Ó