You can quibble with his prescription, but historian Niall Ferguson offers Newsweek readers a pretty good diagnosis of a key problem facing the United States.

What all the Indignant have in common is the refusal to address squarely the problem that nearly all Western countries face. That problem is that the welfare systems that evolved in the mid-20th century are unaffordable under the demographic and economic circumstances of the 21st century. The financial crisis has merely exacerbated what was already a severe structural crisis of public finance, boosting deficits while slowing growth.

The scale of the challenge ranges from the really, really hard to the absolutely impossible. According to the Organization for Economic Cooperation and Development, just to stabilize its debt the U.S. government needs to turn its current primary deficit of 7 percent of gross domestic product into a primary surplus of 1.4 percent. That’s roughly double the fiscal squeeze Greece needs to make.