Why did the Dutch army invade Belgium after Leopold I became king?

The Dutch army invaded Belgium shortly after Leopold I ascended the throne in July 1831 because the Dutch monarch, William I, refused to accept the independence of Belgium.1

Following the Belgian Revolution of 1830, which was sparked by deep-seated political, religious, and economic grievances against William I's rule, Belgium had proclaimed its independence and established a new monarchy. While an international conference in London had recognized Belgium as an independent and neutral state, William I remained unyielding in his opposition to the separation.21

The invasion was a military attempt by the Dutch to reclaim control over the southern territories. Although the Dutch forces initially defeated the Belgian army, the conflict ended when the London Conference authorized intervention by the French army, which compelled the Dutch to withdraw. William I continued to resist the settlement for several years, refusing to officially recognize Belgium's borders until 1839.1