Belgium–Luxembourg border

Dividing the duchy

Luxembourg backed the Belgian revolution against King William I of the Netherlands in 1830, and Belgium claimed the whole grand duchy. In 1831 the great powers decided that William would keep Luxembourg, as part of the German Confederation, but that its French-speaking part would go to Belgium. Belgium accepted; William refused until 1839.^[2]

The Treaty of London of 1839 set out the line. Starting at the French border between Rodange and Athus, it left Arlon and the roads to Longwy and Bastogne to Belgium, passed between named villages to Martelange, followed the deepest channel of the Sûre for a stretch, and then ran north to the Prussian frontier along the old boundary of the Diekirch district. Everything west of the line went to Belgium, everything east stayed with the grand duchy.^[1]

“…leaving to Belgium the road from Arlon to Longwy, the town of Arlon with its district, and the road from Arlon to Bastogne…”^[1]

A neutral neighbour

The same treaty made Belgium an independent and perpetually neutral state. Britain cited it in its ultimatum to Germany on 3 August 1914, before Germany invaded Belgium.^[1]

The remaining grand duchy was run autonomously from the Netherlands from 1839 to 1867. In the 20th century Luxembourg tied its economy closely to Belgium’s, and both were founding members of the Benelux union in 1944.^[2][3]