Former U.S. President Barack Obama, left, delivers his speech at the 16th Annual Nelson Mandela Lecture at the Wanderers Stadium in Johannesburg, South Africa, Tuesday, July 17, 2018. (AP Photo/Th …

Former U.S. President Barack Obama, left, delivers his speech at the 16th Annual Nelson Mandela Lecture at the Wanderers Stadium in Johannesburg, South Africa, Tuesday, July 17, 2018. (AP Photo/Themba Hadebe)