In 1792 the troops of the French republic managed to conquer the Southern Netherlands and Liège, but at the start of 1793 the Austrians reoccupied the country.

A new French military campaign in 1794 put an end to Austrian rule once and for all. France annexed the Southern Netherlands and Liège. The newly integrated territories were imposed the same republican institutions. When Napoleon came to power, Belgium became part of the French empire. Napoleon implemented a wide ranging framework of civil law, which is known as the Code Napoléon. This later became the basis of civil law in Belgium.

During the French period Belgian industry took off. Steam engines and machines were smuggled into Belgium and British immigrants erected factories. Wallonia became the most industrialized region on the continent of Europe. Ghent was the only industrial city in Flanders. The port of Antwerp benefited from the French occupation as the Dutch blockade of the Scheldt was lifted.

The French governement imposed military service on all young males. So many Belgians were forced to fight in the Napoleonic wars. During his reign all political freedom was suspended. In Flanders, use of the Dutch language for any purpose was supressed and the press barred from using it.

After the defeat of Napoleon, the allied powers meeting at the Congress of Vienna (1814-1815) decided to erect a barrier against France by uniting the Northern and Southern Netherlands and the princedom of Liège under the rule of King William I.

William I promoted the industrialization of the country. To support mechanised industry, he embarked on large scale infrastructural development and provided financial assistance to enterpreneurs. His measures met with the approval of the industrialists of Wallonia and Ghent. In the rest of Flanders non-mechanised rural industry suffered greatly from the competition represented by new factories, which caused some dissatisfaction.

The catholic church supported this rural opposition because it refused to accept a protestant king. Moreover, William I wanted to limit clerical power, which resulted in catholic distrust.

The king was not so liberal in political matters. He refused to devolve power to government ministers, which was what the Liberals demanded. In linguistic matters, he wanted to impose Dutch in Flanders. The young Walloon and Flemish upper-classes, which spoke French, were afraid that their career opportunities would be affected.

In 1828 Catholics and young Liberals formed a Union which had a joint programme. Finally, in 1830, tensions in the south rose to such an extent that the area broke away from the north and formed a new independent Belgian state.