Go back far enough, and the present-day Benelux countries were lands of the Dukes of Burgundy (and there were several iterations/variants of “Burgundy”, taking in parts of Switzerland and Germany). By dynastic marriage and the politics of the Holy Roman Empire, they passed to the Emperor Charles V, and then to Spain when he divided the Spanish and Austrian parts of the Empire between his sons.
The southern provinces of the Netherlands stayed under Spanish rule when the northern provinces won independence, finally recognised in the resolution of the Thirty Years’ War. But barely 60 years later, the accidents of infertility in the Spanish monarchy led to war in the Netherlands, as Louis XIV tried to impose his candidate for the Spanish monarchy. In the eventual settlement of that war, what is now Belgium became the Austrian Netherlands - and hence a base for the powers resisting the French revolution (and Britain got Gibraltar, but that’s another story).
Come the Napoleonic wars, the final settlement gave Belgium to a reunited Netherlands (whose new king also became Grand Duke of Luxembourg, but that too is another story). That settlement lasted less than twenty years, till the risings of 1830 and the subsequent short war before Belgian independence was recognised, and dynastic definitions of a country really started giving way to civic and/or ethnic nationalism.
You can see why they called it the “cockpit of Europe” - and geography made it so again in 1914 and 1940, as the backdoor to France.