When America’s housing market turned, a chain reaction exposed fragilities in the financial system. Pooling and other clever financial engineering did not provide investors with the promised protection. Mortgage-backed securities slumped in value, if they could be valued at all. Supposedly safe CDOs turned out to be worthless, despite the ratings agencies’ seal of approval. It became difficult to sell suspect assets at almost any price, or to use them as collateral for the short-term funding that so many banks relied on. Fire-sale prices, in turn, instantly dented banks’ capital thanks to “mark-to-market” accounting rules, which required them to revalue their assets at current prices and thus acknowledge losses on paper that might never actually be incurred.