In 1796 he married Princess Louise of Prussia, the second daughter of Prince Augustus Ferdinand of Prussia and hence a niece of the late Prussian king Frederick the Great. His new family convinced him that he should be a mediator between the Poles living under the Third Partition after the failed Kościuszko Uprising and the Prussian authorities in Berlin. Fluctuating between Berlin, Warsaw and Saint Petersburg, Radziwiłł developed the idea of making the province of South Prussia the nucleus of a renewed Polish kingdom, ruled by the Prussian king in personal union.