Even as the economy continues to struggle, much of Wall Street is minting money — and looking forward again to hefty bonuses.

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Many Americans wonder how this can possibly be. How can some banks be prospering so soon after a financial collapse, even as legions of people worry about losing their jobs and their homes?

It may come as a surprise that one of the most powerful forces driving the resurgence on Wall Street is not the banks but Washington. Many of the steps that policy makers took last year to stabilize the financial system — reducing interest rates to near zero, bolstering big banks with taxpayer money, guaranteeing billions of dollars of financial institutions’ debts — helped set the stage for this new era of...