When George Mattson met Ed Bastian in the late 1990s, he couldn’t have predicted their relationship would eventually help shape how millions of people travel around the world. At the time, Mattson was a young banker at Goldman Sachs. He’d just taken on responsibility for major client relationships in the Southeast, including Delta Air Lines. Bastian, who’d recently joined Delta from PepsiCo, was then the airline’s controller. They were both in their early 30s, early in their leadership journeys and, at least on paper, just two more names on each other’s growing contact lists. “In that period of my career, I was building relationships with hundreds of different people,” Mattson recalls. “You never know which relationships you build over time will become more central to your future activities.” At first, he and Bastian didn’t have many reasons to overlap day to day. As Bastian was promoted to CFO, however, he took on responsibility for Delta’s international partnerships, mergers and...