On October 4, 1830, the provisional government sitting in Brussels declared the Belgian provinces independent of the Kingdom of the Netherlands, ending a union the Congress of Vienna had imposed fifteen years earlier. The declaration covered the provinces that had been the Austrian Netherlands, apart from Luxembourg. Fighting had begun in August.
It started at the opera. On August 25, during a performance of Auber's La Muette de Portici at the Theatre de la Monnaie, the audience came out singing and joined a crowd attacking government buildings, and within days the middle-class civic guard had gone over to the rioters. King William I sent troops under his son, and they got into Brussels on September 23 and were fought out of it over four days.
The union had never been comfortable. The southern provinces were Catholic and largely French-speaking, the north Protestant and Dutch-speaking, and William I governed as if the difference did not matter, imposing Dutch as the official language and dividing public offices in a way that left the south, with two thirds of the population, under-represented. The south also had the industry, and it wanted tariffs.
The powers decided the outcome. Britain, France, Prussia, Austria and Russia met in London from November and produced a protocol recognising Belgian independence in January 1831, on condition that the country be neutral and that the neutrality be guaranteed. Leopold of Saxe-Coburg accepted the throne in July 1831. William I refused. He invaded in August, and French troops pushed him out in ten days.
The final separation came in 1839, when the Netherlands recognised Belgium under the Treaty of London and Luxembourg was divided. The guarantee of that neutrality is what Britain cited in August 1914 when German troops crossed it. The country's linguistic split has shaped its politics ever since: the Flemish and the Walloon parties govern in coalition, and there are no national parties left to speak of.