Why did Castlereagh want Prussia to control territory along the Rhine River?

At the Congress of Vienna, Viscount Castlereagh and other allied diplomats sought to establish a stable balance of power in Europe following the defeat of Napoleon. A primary objective of the Congress was to prevent any future French aggression.12

To achieve this, the victorious powers decided to strengthen the nations bordering France, effectively creating a series of buffer states. By granting Prussia control over territory along the Rhine River—specifically the Rhineland and Westphalia—the Congress established Prussia as the primary defender of Germany’s western boundary. This strategic placement ensured that a major power was positioned to guard against potential French expansionism, contributing to the long-term stability and relative peace that characterized Europe for the following half-century.21