Original: Otto's lack of success owed partially to his being the brother of Emil von Zalewski, who on August 17, 1891 while leading a colonial force against Wehe insurgents in Tanzania was speared to death while his force was annihilated in the battle of Rugarto.
Simplified: His father's lack of success was partly because he was the brother of Emil von Zalewski,  the leader of a colonial force fighting in Tanzania.