Of Celtic origin, Alsace became part of the Roman province of Upper Germany (see Gaul). It fell to the Alemanni (5th cent.) and to the Franks (496). The Treaty of Verdun (843; see Verdun, Treaty of) included it in Lotharingia; the Treaty of Mersen (870) put it in the kingdom of the East Franks (later Germany). The 10 chief cities of Alsace gained (13th cent.) virtual independence as free imperial cities. The remainder of the region was divided into fiefs with the exception of Upper Alsace, where the Hapsburg family consolidated its original holdings.
Alsace became a center of the Reformation (although the rural areas remained generally Catholic). The Treaty of Westphalia (1648) transferred all Hapsburg lands in Alsace to France. Lower Alsace was conquered (1680–97) by Louis XIV of France; the Treaty of Ryswick (1697) confirmed French possession. The Edict of Nantes (1685), promulgated before the annexation of Alsace, could not be revoked; therefore religious worship remained free. In 1798 the city of Mulhouse voted to join France.
In 1871, as a result of the Franco-Prussian War, all Alsace (except Belfort) was annexed by Germany. With part of Lorraine, it formed the "imperial land" of Alsace-Lorraine, held in common by all the German states. Many inhabitants emigrated to France rather than submit to a policy of Germanization. Clamor for the return of Alsace-Lorraine became the chief rallying force for French nationalism and was a major cause of the armaments race that led to World War I. France's recovery (1918) of this territory was confirmed by the Treaty of Versailles (1919).
After the decline of early enthusiasm over the reunion with France, a strong particularist movement gained ground, demanding cultural and even political autonomy. The movement received impetus from recurrent efforts by the French government to end the Concordat of 1801, which had remained valid in Alsace-Lorraine although it had been ended in the rest of France in 1905. In 1940, German troops occupied Alsace; a large part of the population had already been evacuated to central France. Alsace was treated as a part of Germany. French and American troops recovered (Jan., 1945) Alsace for France and were generally hailed as liberators.