Roughly speaking, in 1830 the Great Powers agree to partition the Catholic areas of the Kingdom of Netherland then in rebellion instead of setting them up as the united and indepedent neutral buffer state of Belgium. France gets Wallon areas, Netherlands keeps Flemish areas, Prussia gets Luxemburg, Eastern Liege-Namur, and Maastricht.
How would that affect history ? For one thing, no Belgium means other European countries colonize the Congo basin (likely the UK, they were expanding from Rhodesia, or maybe Germany), and maybe partition the area, which means no republic of Congo as we know it, no troubles in the '60s, no wars in equatorial Africa in the 1990s. Another thing, no neutral Belgium state in 1914 might mean no Schliffen Plan: Germany adopts an East First strategy, which butterflies the Entente UK away.