Sometime in April, after 132 years, the General Electric Company will be no more. GE, once the world’s greatest conglomerate, will be split into its component parts. The aerospace and energy businesses will be spun off, following the healthcare division that C.E.O. Larry Culp amputated last year. Time will tell whether breaking up GE and abandoning the conglomerate structure was a good thing or a bad thing. But one thing is for sure: When Jack Welch ran GE, it was often the most admired and most valuable company in the world. How it reached this fateful denouement is a saga both simple and complex.

Alas, things started going awry at GE one business day after Jeff Immelt took over from Jack—on September 10, 2001. Jeff once told me that was the only good day he had in his first year running the company. The next morning, when all hell broke loose, GE lost some $1 billion in a matter of hours. GE, after all, had made the engines on the jets involved in 9/11. GE had reinsured 7 World Trade Center. Two of its employees were killed. At the time, GE also owned NBC, which ran essentially commercial-free for three days, costing the company several hundred million dollars. Ironically, Jeff himself was in Seattle that morning, having flown out the night before, for a long-scheduled meeting with the C.E.O. of Boeing. He was on the StairMaster when the planes hit the two towers.