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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 of German territory.
The territories of the Left Bank of the Rhine, which French troops had conquered in 1795, had been returned to German (mostly Prussian) control after the 1815 Congress of Vienna, forming the Rhine Province. After a diplomatic defeat in the Oriental Crisis of 1840 France shifted its focus to the Rhine, and the French government, led by Adolphe Thiers, restated its claim to areas on the left bank, to re-establish the Rhine as a natural border. These claims reinforced ressentiment among the Germans against the French, and increased nationalism on both sides. New nationalist songs were written in France and Germany and achieved huge popularity, most famously the German songs "Die Wacht am Rhein", "Der Deutsche Rhein" and the "Lied der Deutschen", the national anthem of Germany since 1922.
In the end the crisis passed without war breaking out, though it again established the importance of the Rhine in European culture and politics.
The Greek War of Independence and the Russo-Turkish War (1828–29) had considerably weakened the Ottoman Empire, and after Sultan Mahmud II refused to acknowledge Muhammad Ali Pasha (viceroy of the Ottoman territory Egypt) as the Stadtholder of Syria, Egyptian troops moved into Palestine and Syria, reaching Anatolia by 1832.
France had used the Turkish defeat in the Greek War of Independence to occupy Algeria, in 1830, and regarded Muhammad Ali Pasha as an ideal partner, supporting him in his effort to rid himself once and for all of his Ottoman overlord. France's political goals were to bring Mediterranean Africa beyond Suez under its sphere of influence.
When Muhammad Ali won a victory over the Ottoman sultan in 1839, the Oriental Crisis of 1840 was born. The United Kingdom, Russia, Prussia, and Austria considered that the fall of the Ottoman Empire would entail unacceptable consequences, and signed the 1840 Convention of London, which would stabilize the Empire and demanded from France that it withdraw its support for Egypt's claims. British military assistance for the Ottomans forced Muhammad Ali Pasha to give up Syria and Palestine and limit his authority to Egypt, which remained under Ottoman overlordship (in return, his rule became hereditary).
But before the crises over the Dardanelles and the Rhine could escalate into a European war, the Thiers government, whose politics of prestige had started the crisis in the first place, was brought down. A new cabinet, with Guizot as Secretary of State, sailed a more conciliatory course, and the London Straits Convention (13 July 1841) settled the matter of the Dardanelles, defusing tension in Europe.
In Germany the consequences of the Rhine crisis were described thus by Heinrich Heine: "Thiers drummed our fatherland into this great movement which awakened political life in Germany; Thiers brought us back on our feet as a nation." The crisis led the German Confederation to extend the fortifications of Mainz, Ulm, and Rastatt, while the Kingdom of Bavaria built the fortress at Germersheim.
- Gruner, Wolf D. "Der Deutsche Bund, die deutschen Verfassungsstaaten und die Rheinkrise von 1840. Überlegungen zur deutschen Dimension einer europäischen Krise (The German Confederation, the German constitutional states and the Rhine crisis of 1840. Considerations on the German dimension of a European crisis)." Zeitschrift für bayerische Landesgeschichte, Nr. 53 (1990), p. 51–78.
- Püschner, Manfred. "Die Rheinkrise von 1840/41 und die antifeudale Oppositionsbewegung (The Rhine crisis of 1840/41 and the anti-feudal opposition movement)." Schriften des Zentralinstituts für Geschichte, Nr. 50 (1977), p. 102–133.
- Müller, Frank Lorenz. "Der Traum von der Weltmacht. Imperialistische Ziele in der deutschen Nationalbewegung von der Rheinkrise bis zum Ende der Paulskirche (The dream of a world power. Imperialist goals in the German national movement of the Rhine crisis until the end of the Frankfurt Parliament)." Jahrbuch der Hambach Gesellschaft 6 (1996/97), p. 99–183.