Leopold II of Belgium was king.
The Congo Arab war ( also known as the Congolese-Arab war and the Belgo-Arab War ) took place in what is now the Democratic Republic of the Congo between the forces of Belgian King Leopold II 's Congo Free State and various Zanzibari `` Arab '' slave traders led by Sefu bin Hamid , the son of Tippu Tip . Leopold II ( 9 April 1835 -- 17 December 1909 ) reigned as the second King of the Belgians from 1865 to 1909 . he succeeded his father to the Belgian throne in 1865 and reigned for 44 years until his death - the longest reign of any Belgian monarch .