Personal union of the Netherlands and Luxembourg

Luxembourg had historically been part of the Holy Roman Empire and later came under French control during the French Revolutionary Wars.^[1][2] Following the defeat of Napoleon, European powers reorganized territorial arrangements at the Congress of Vienna.^[3] As part of this settlement, the Sovereign Prince of the Netherlands, who became William I of the Netherlands, was granted Luxembourg as compensation for the loss of ancestral Nassau territories to Prussia.^[4][5] Luxembourg was elevated to the status of a Grand Duchy. The territory became a member of the German Confederation, giving it a dual political character: linked dynastically to the Netherlands but constitutionally associated with the German states.^[6][7]

Despite sharing a monarch, the Netherlands and Luxembourg maintained distinct political systems.^[8][9] Luxembourg's strategic fortress in Luxembourg City was garrisoned by Prussian troops as part of the German Confederation's defense arrangements.^[10][11]

Per the Treaty of London (1839), William I ceded the western portion of Luxembourg to the nascent country of Belgium, which had seceded from the Netherlands nine years earlier. As Luxembourg had been a part of the German Confederation, the latter insisted that the Netherlands compensate for the Confederation's losses elsewhere. Thus the Dutch created the Duchy of Limburg, and William I thus became monarch of all three sovereign states.^[12] This union lasted until 1866, when the Seven Weeks' War between Austrian Empire and Prussia effectively dissolved the German Confederation. The Second Treaty of London clarified that albeit Limburg was part of the Confederation, it was a possession of the Dutch monarchy, and thus was reinculated into the Netherlands on 23 August 1866; the Duchy today exists as the province of Limburg, the southernmost of the twelve Dutch provinces.

The style "Duchy of Limburg" continued to be used in some official capacities until February 1907. An idiosyncrasy maintained from the Duchy is that the King's commissioner for Limburg province is still informally addressed as "Governor" in Limburg, although his formal style does not differ from that used in the other provinces.

Tensions surrounding Luxembourg's status culminated in the Luxembourg Crisis of 1867, when Napoleon III of France attempted to purchase the territory from William III.^[13][14] Prussia objected strongly to Imperial France's actions due to Luxembourg's strategic importance.^[15] The dispute raised fears of war between France and Prussia.^[16] The crisis was resolved by the Second Treaty of London, which provided that:^[17][18]

1. Luxembourg would become a permanently neutral state.
2. The fortress of Luxembourg City would be dismantled.
3. Prussian troops would withdraw.
4. The personal union with the Netherlands would continue.

The personal union ended upon the death of William III on 23 November 1890. As the King-Grand Duke left only a daughter, Wilhelmina, and Luxembourg followed the Nassau Family Pact of 1783, which required a male dynast, the personal union was terminated. Wilhelmina became Queen of the Netherlands, whereas the Luxembourgish throne passed to Adolphe, Duke of Nassau, from the Nassau-Weilburg branch of the family.^[19][20]

1. William I 1815–1840
2. William II 1840–1849
3. William III 1849–1890

The personal union contributed to Luxembourg's development as a distinct European state while maintaining dynastic ties to the Dutch monarchy.^[21][22] The separation in 1890 reinforced Luxembourg's independence and established the House of Nassau-Weilburg as its reigning dynasty.^[23][24]