You needed models to create the intricate network of bonds based on the homeowners’ payments, models to predict prepayment rates, and models to predict defaults. You needed the Internet to sail these bonds back and forth across the world, massaging their content to fit an investor’s needs at a moment’s notice. Add to all this the complacency, greed, entitlement, and callous stupidity that characterized banks in post-2001 America, and you have a recipe for disaster.