Many countries need to strengthen their institutions and face up to rising threats like extremist ideology

Not long ago, Amina J. Mohammed, the U.N.’s deputy secretary-general, witnessed firsthand democracy in action in Africa. After Muhammadu Buhari won Nigeria’s presidential elections in March 2015, he appointed her environment minister. The election marked Nigeria’s first peaceful transition of power in decades; since attaining independence in 1960, the country’s political history had been marked by…