Signed in London by the major European powers, the treaty resolved years of dispute following Belgium's split from the Netherlands by dividing the old Grand Duchy of Luxembourg: the French-speaking western portion joined Belgium as its province of Luxembourg, while the remainder became a fully sovereign Grand Duchy tied personally to the Dutch crown. The settlement fixed Luxembourg's modern borders and cemented its unusual status as an independent state.
Luxembourg's road to nationhood ran through someone else's revolution. When Belgian provinces rose against Dutch rule in 1830, the Grand Duchy of Luxembourg, then held in personal union with the Netherlands, was swept up in the chaos as most of its population and its capital sided with the Belgian rebels.
The Great Powers, wary of redrawing Europe's map too casually after Napoleon, spent nearly a decade negotiating a fix. The Treaty of London, signed April 19, 1839, split the duchy roughly in half. The western, largely French-speaking part became Belgium's province of Luxembourg. The eastern remainder, smaller and poorer, kept the name Grand Duchy of Luxembourg and remained tied to the Dutch king personally rather than to the Netherlands as a state, an odd constitutional arrangement that nonetheless guaranteed its independence.
The treaty's borders have held essentially unchanged since, making Luxembourg one of Europe's most stable territorial success stories, even as its independence was almost an accident of diplomatic bookkeeping. The tiny state's neutrality, also guaranteed by the treaty, would be violated twice by Germany in the twentieth century, but its borders survived intact, a rare thing on a continent that spent the next hundred years redrawing itself repeatedly through war.
Key people: William I of the Netherlands