The Huguenot exodus from France (XVII century)
In 1685, Louis XIV revoked the Edict of Nantes and declared Protestantism to be illegal in the Edict of Fontainebleau. After this, Huguenots (with estimates ranging from 200,000 to 1,000,000) fled to surrounding Protestant countries: England, the Netherlands, Switzerland, Norway, Denmark and Prussia — whose Calvinist Great Elector Frederick William welcomed them to help rebuild his war-ravaged and underpopulated country. Many went to the Dutch colony at the Cape (South Africa) where they were instrumental in establishing a wine industry.
The exodus of Huguenots from France created a brain drain, as many Huguenots had occupied important places in society, from which the kingdom did not fully recover for years.
 Antisemitism in pre-WWII Europe
Antisemitic feelings and laws in Europe through the 1930's and 1940's, culminating in the holocaust, caused the emigration of many scientists to the United States. Notable examples are:
Albert Einstein (emigrated permanently to the United States in 1933)
Enrico Fermi (1938)
Niels Bohr (1943)
In addition to the antisemitic conditions, Nazi political opposition against the liberal, socialists in Germany contributed to another kind of emigration. The Bauhaus, perhaps the most important arts and design school of the 20th century, was forced to close down during the Nazi regime because of their liberal and socialist leanings, which the Nazis considered was degenerate art. The school had already been shut down in Weimar because of its political stance but moved to Dessau prior to the closing. Following this abandonment, two of the three pioneers of Modern architecture, Mies Van Der Rohe and Walter Gropius, left Germany for America (while Le Corbusier stayed in France). Along with them, they brought the European modern movement to the rather unaware American public and fostered the international style in architecture and design. They helped to transform design...