In 1830, Belgium had broken away from the Netherlands.
In February 1831, the National Congress adopted a constitution which, at that time, was considered as progressive.
It was on 21 July 1831 that Leopold of Saxe-Coburg swore allegiance to the Belgian constitution in the Sint Jacobs Church on the Coudenberg in Central Brussels.
Leopold I (photo above) thus became the first king of the Belgians. The great powers at that time recognised the Belgian independence.
Many people, including Belgium's leading politicians, can't remember exactly why 21 July is our National Day.
In 2007, (now former) Prime Minister Guy Verhofstadt (Flemish liberal) was only able to answer after several minutes of deep reflection. However, he did better than his successor Yves Leterme (Flemish Christian democrat) and Walloon PM Rudy Demotte (Francophone socialist), who could not answer.