Why did Brussels share capital status with The Hague from 1815 to 1830?

Following the defeat of Napoleon I in 1815, the Congress of Vienna established the Kingdom of the Netherlands by reuniting the northern and southern provinces of the Low Countries, which had been separated since the 16th century.12

This new state was created by the Allied powers to serve as a strategic barrier against potential French expansion. During this 15-year period (1815–1830), Brussels and The Hague shared the status of capital to accommodate the administrative needs of the newly unified kingdom.21

However, the union was marked by significant cultural, religious, and political friction between the two regions. The north was predominantly Protestant and commercial, while the south was Roman Catholic, increasingly industrial, and largely French-speaking among the elite. These tensions, exacerbated by the authoritarian policies of King William I, ultimately led to the Belgian Revolution of 1830, which resulted in the independence of Belgium and the establishment of Brussels as its sole capital.231