Ten years ago, amid a worsening subprime mortgage crisis, the U.S. government did what few have dared: It allowed a major global investment bank, Lehman Brothers Holdings Inc., to file for bankruptcy. Within days, the shock waves crippled the nation's largest insurer, triggered a run on money-market funds, and accelerated a cash crunch that would ultimately destroy millions of jobs. Only by pledging trillions of dollars to prop up the financial system, and spending hundreds of billions more on fiscal stimulus, did the government manage to prevent the worst economic disaster since the Great Depression from becoming the worst ever.