When the Treaty of the 24 Articles was applied in 1839, the Grand Duchy of Luxembourg, founded in 1815 by the Vienna Congress, was divided up between King-Grand Duke William I and the new Belgian state: the French-speaking parts and the Arlon area became part of Belgium, as the Belgian province of Luxembourg, and the rest, the area of today's Grand Duchy, remained with William I and retained its complicated status (personal union with the Netherlands, membership of the German Confederation and a Prussian garrison for the federal Fortress of Luxembourg).
But because the population of the Grand Duchy had greatly sympathised with the Belgians during the Belgian Revolution (1830-1839), due to William I's clumsy and unfair governing of Luxembourg, Belgium over the course of the years tried again and again to "regain" the reduced Grand Duchy, as something that had been unfairly taken from them in 1839.
In the initial phase, Belgium did not engage in a directly annexationist policy. It was not yet strong enough to do so, and the new state had to prove itself stable and viable. But Belgium did secure direct commercial advantages for the Grand Duchy with the Loi de Faveur of 6 June 1839, and any inhabitant of the Grand Duchy who moved to Belgium, could demand and receive Belgian citizenship.
In 1866 the Kingdom of Prussia dealt the Austrian Empire a decisive military defeat at Königgrätz in Bohemia. The German Confederation was dissolved, and Austria was forced out of German affairs, to the advantage of Prussia. The Prussians, however, still left their garrison in Luxembourg. When the King-Grand Duke William III offered to sell his Grand Duchy to French Emperor Napoleon III in 1867, a Franco-Prussian war almost ensued, as public opinion in the Prussian-dominated German territories did not want to see France in possession of the Grand Duchy, where "Luxembourgish German" was spoken and which had belonged to the German Confederation until 1866 and the Zollverein since 1842.
In the context of the crisis, which was finally solved in 1867 at the conference of London, by making Luxembourg neutral, the Belgian King Leopold II declared: "The annexation of the Grand Duchy of Luxembourg to Belgium would be the glory of my reign" (" L'annexion du Grand-Duché de Luxembourg à la Belgique serait la gloire de mon règne "). This wish, for the "return to the mother country of the brothers who were left behind in 1839" (Belgian historian R. Demoulin 1968), was not fulfilled.
When King-Grand Duke William III died in 1890 without male heirs, Belgian saw an opportunity to "recover" Luxembourg. Through applying the family pact of Nassau, however, the Grand Duchy received a new dynasty: the former Duke of Nassau, Adolphe, became Grand Duke of Luxembourg, and the Belgians saw their opportunity pass.
Under Grand Duke William IV, who had no male heirs but six daughters, and who was in ill health, there were renewed speculations in Belgium. But in 1907 the line of succession of Lxembourg was change by law (Salic Law was abolished) and William IV's oldest daughter, Princess Marie-Adélaide, became Grand Duchess in 1912 after her father's death.
In World War I, and in 1919, Belgian annexationism was on the agenda. Belgian propaganda made great efforts to try and link Luxembourg to Belgium, under the pretext that Luxembourg, which was neutral and had not real army, had not done its duty in the war, and that Grand Duchess Marie-Adélaide had sympathies for the German occupiers. The propagandists of a "Grande Belgique", including Gaston Barbanson, Pierre Nothomb and Fernand Neuray, argued for simply annecting the country. The territory of the Grand Duchy would then have become a new province of Belgium, the province of East-Luxembourg. If that were not possible, then a personal union between the Belgian Kingdom and the Grand Duchy of Luxembourg would also have been acceptable: the King of the Belgians, Albert I, would simply have become Grand Duke of Luxembourg. The minimum that the annexationists would accept was an economic union. Since France, however, was also interested in Luxembourg (steel industry, railway network, ...), and Belgium did not want to provoke a confrontation with the French, it came to an agreement: France left Luxembourg to Belgium, but Belgium made a secret defence agreement with France.
In Luxembourg, where a grave political crisis was on-going, there was a double referendum on 28 September 1919:
- In the political section, the population - having only just been granted universal suffrage - chose to maintain Grand Duchess Charlotte, who had succeeded Marie-Adelaide in 1919 after her abdication
- In the economic part, voters expressed a preference for cooperation with France.
The option of a personal union with Belgium, or an economic union, was rejected. This was a defeat for the Belgian annexationists. Due to the Franco-Belgian agreement mentioned above, Belgian-Luxembourgish negotiations came about. In the end, Luxembourg remaind independent, with its own dynasty; economically, after long and tough negotiations, the "Union économique belgo-luxembourgeoise" (UEBL) was founded and ratified in 1922.