On paper, Luxembourg became a state distinct from the Netherlands in 1815. In reality, however, William I made no such distinction. He governed the Grand Duchy as though it were the 18th province of his kingdom.
The Dutch fundamental law was applied to Luxembourg, Luxembourg delegates sat in Dutch institutions and Dutch was taught at school. While the Luxembourg people did not oppose this, the economic and in particular fiscal policies of the Dutch regime gave rise to a growing discontent among the population.
It was not surprising therefore that, when the Belgian Revolution started in 1830, the inhabitants of the Grand Duchy joined the Belgian insurgents. A great number of Luxembourg volunteers left for Brussels to join the army of the patriots. Following Belgium’s declaration of independence on 4 October 1830, several Luxembourg representatives sat in the constituent assembly, and later in the institutions of the young Belgian state. Only the capital of the Grand Duchy remained under Dutch control, since it was protected by the Prussian garrison.
Keen to extinguish the revolutionary fire, the great powers decided to separate the Belgians and the Dutch by creating the Kingdom of Belgium, while dividing the Grand Duchy of Luxembourg between the two adversaries (Treaty of the Twenty-Four Articles dated 14 October 1831). The Belgian parliament accepted this decision, while William I refused to do so.
During the following eight years, the Grand Duchy had a double administration. The power of the Dutch was limited to the fortress-city, while the rest of the country was under Belgian authority.
Eventually, William I agreed to the decision of the great powers. The Treaty of London of 28 April 1839 imposed the division. Henceforth, there were two Luxembourgs:
- the Grand Duchy of Luxembourg, which remained under the rule of the Orange-Nassau,
- and Belgian Luxembourg, which became a province of Belgium.
The dividing line more or less followed the linguistic border, except in the region of Arlon. The treaty of 1839 defined the borders of the Grand Duchy, which have not changed since.