Complex sentence: In 1831, a 50,000-strong Dutch force was built up in Brabant along the Belgian border, allegedly to protect the frontier, under the command of the Prince of Orange (the future King William II).
Split into simpler sentences: In 1831, a 50,000-strong Dutch force was built up in Brabant along the Belgian border, allegedly to protect the frontier. It was commanded by the Prince of Orange (the future King William II).