Rhine crisis
The Rhine crisis of 1840 was a diplomatic crisis between the Kingdom of France and the German Confederation, caused by the demand by French minister Adolphe Thiers that the river Rhine be reinstated as France's border in the east, at a loss of some 32,000 km2 (12,000 sq mi) of German territory. The territories of the Left Bank of the Rhine, which French troops had conquered in 1795 and formally annexed in 1797, had been returned to German (mostly Prussian) control after the 1815 Congress of Vienna, forming the Rhine Province.