When Detroit filed for bankruptcy in 2013, the city’s emergency-financial team said that high levels of retirement debt could prevent them from rescuing the Motor City’s finances. Detroit had been in economic decline for decades, and the pension problem—including billions of dollars in bonuses handed out while the city was hurtling toward insolvency—was just one part of the depressing financial picture. Dallas, by contrast, has been one of the fastest-growing American cities in recent years. Becoming a magnet for investment and opportunity, however, hasn’t protected the Texas city from experiencing its own Detroit-style financial crisis. Dallas’s retirement system for cops...