Piedmontese soldier, jurist, and an authority on the law of war who is considered one of the founders of modern international law.

After serving as commander in chief of the army of the Holy Roman Empire in Piedmont, Belli was appointed (1560) a councillor of state by Emmanuel Philibert, Duke of Savoy. His book De re militari et de bello (1563) was for its time an unusually thorough treatment of military law and the rules for conducting war.