Beginning in 2004 a series of developments portended the coming crisis, though very few economists anticipated its vast scale. Over a two-year period (June 2004 to June 2006) the Fed raised the federal funds rate from 1.25 to 5.25 percent, inevitably resulting in more defaults from subprime borrowers holding adjustable-rate mortgages (ARMs). Partly because of the rate increase, but also because the housing market had reached a saturation point, home sales, and thus home prices, began to fall in 2005. Many subprime mortgage holders were unable to rescue themselves by borrowing, refinancing, or selling their homes, because there were fewer buyers and because many mortgage holders now owed more on their loans than their homes were worth (they were “underwater”)—an increasingly common phenomenon as the crisis developed. As more and more subprime borrowers defaulted and as home prices continued to slide, MBSs based on subprime mortgages lost value, with dire consequences for the portfolios of many banks and investment firms. Indeed, because MBSs generated from the U.S. housing market had also been bought and sold in other countries (notably in western Europe), many of which had experienced their own housing bubbles, it quickly became apparent that the trouble in the United States would have global implications, though most experts insisted that the problems were not as serious as they appeared and that damage to financial markets could be contained.

By 2007 the steep decline in the value of MBSs had caused major losses at many banks, hedge funds, and mortgage lenders and forced even some large and prominent firms to liquidate hedge funds that were invested in MBSs, to appeal to the government for loans, to seek mergers with healthier companies, or to declare bankruptcy. Even firms that were not immediately threatened sustained losses in the billions of dollars, as the MBSs in which they had invested so heavily were now downgraded by credit-rating agencies, becoming “toxic” (essentially worthless) assets. (Such agencies were later accused of a severe conflict of interest, because their services were paid for by the same banks whose debt securities they rated. That financial relationship initially created an incentive for agencies to assign deceptively high ratings to some MBSs, according to critics.) In April 2007 New Century Financial Corp., one of the largest subprime lenders, filed for bankruptcy, and soon afterward many other subprime lenders ceased operations. Because they could no longer fund subprime loans through the sale of MBSs, banks stopped lending to subprime customers, causing home sales and home prices to decline further, which discouraged home buying even among consumers with prime credit ratings, further depressing sales and prices. In August, France’s largest bank, BNP Paribas, announced billions of dollars in losses, and another large U.S. firm, American Home Mortgage Investment Corp., declared bankruptcy.

In part because it was difficult to determine the extent of subprime debt in any given MBS (because MBSs were typically sold in pieces, mixed with other debt, and resold in capital markets as new securities in a process that could continue indefinitely), it was also difficult to assess the strength of bank portfolios containing MBSs as assets, even for the bank that owned them. Consequently, banks began to doubt one another’s solvency, which led to a freeze in the federal funds market with potentially disastrous consequences. In early August the Fed began purchasing federal funds (in the form of government securities) to provide banks with more liquidity and thereby reduce the federal funds rate, which had briefly exceeded the Fed’s target of 5.25 percent. Central banks in other parts of the world—notably in the European Union, Australia, Canada, and Japan—conducted similar open-market operations. The Fed’s intervention, however, ultimately failed to stabilize the U.S. financial market, forcing the Fed to directly reduce the federal funds rate three times between September and December, to 4.25 percent. During the same period, the fifth largest mortgage lender in the United Kingdom, Northern Rock, ran out of liquid assets and appealed to the Bank of England for a loan. News of the bailout created panic among depositors and resulted in the first bank runs in the United Kingdom in 150 years. Northern Rock was nationalized by the British government in February 2008.

The crisis in the United States deepened in January 2008 as Bank of America agreed to purchase Countrywide Financial, once the country’s leading mortgage lender, for $4 billion in stock, a fraction of the company’s former value. In March the prestigious Wall Street investment firm Bear Stearns, having exhausted its liquid assets, was purchased by JPMorgan Chase, which itself had sustained billions of dollars in losses. Fearing that Bear Stearns’s bankruptcy would threaten other major banks from which it had borrowed, the Fed facilitated the sale by assuming $30 billion of the firm’s high-risk assets. Meanwhile, the Fed initiated another round of reductions in the federal funds rate, from 4.25 percent in early January to only 2 percent in April (the rate was reduced again later in the year, to 1 percent by the end of October and to effectively 0 percent in December). Although the rate cuts and other interventions during the first half of the year had some stabilizing effect, they did not end the crisis; indeed, the worst was yet to come.

By the summer of 2008 Fannie Mae (the Federal National Mortgage Association) and Freddie Mac (the Federal Home Loan Mortgage Corporation), the federally chartered corporations that dominated the secondary mortgage market (the market for buying and selling mortgage loans) were in serious trouble. Both institutions had been established to provide liquidity to mortgage lenders by buying mortgage loans and either holding them or selling them—with a guarantee of principal and interest payments—to other banks and investors. Both were authorized to sell mortgage loans as MBSs. As the share of subprime mortgages among all home loans began to increase in the early 2000s (partly because of policy changes designed to boost home ownership among low-income and minority groups), the portfolios of Fannie Mae and Freddie Mac became more risky, as their liabilities would be huge should large numbers of mortgage holders default on their loans. Once MBSs created from subprime loans lost value and eventually became toxic, Fannie Mae and Freddie Mac suffered enormous losses and faced bankruptcy. To prevent their collapse, the U.S. Treasury Department nationalized both corporations in September, replacing their directors and pledging to cover their debts, which then amounted to some $1.6 trillion.

Later that month the 168-year-old investment bank Lehman Brothers, with $639 billion in assets, filed the largest bankruptcy in U.S. history. Its failure created lasting turmoil in financial markets worldwide, severely weakened the portfolios of the banks that had loaned it money, and fostered new distrust among banks, leading them to further reduce interbank lending. Although Lehman had tried to find partners or buyers and had hoped for government assistance to facilitate a deal, the Treasury Department refused to intervene, citing “moral hazard” (in this case, the risk that rescuing Lehman would encourage future reckless behaviour by other banks, which would assume that they could rely on government assistance as a last resort). Only one day later, however, the Fed agreed to loan American International Group (AIG), the country’s largest insurance company, $85 billion to cover losses related to its sale of credit default swaps (CDSs), a financial contract that protects holders of various debt instruments, including MBSs, in the event of default on the underlying loans. Unlike Lehman, AIG was deemed “too big to fail,” because its collapse would likely cause the failure of many banks that had bought CDSs to insure their purchases of MBSs, which were now worthless. Less than two weeks after Lehman’s demise, Washington Mutual, the country’s largest savings and loan, was seized by federal regulators and sold the next day to JPMorgan Chase.

By this time there was general agreement among economists and Treasury Department officials that a more forceful government response was necessary to prevent a complete breakdown of the financial system and lasting damage to the U.S. economy. In September the George W. Bush administration proposed legislation, the Emergency Economic Stabilization Act (EESA), which would establish a Troubled Asset Relief Program (TARP), under which the Secretary of the Treasury, Henry Paulson, would be authorized to purchase from U.S. banks up to $700 billion in MBSs and other “troubled assets.” After the legislation was initially rejected by the House of Representatives, a majority of whose members perceived it as an unfair bailout of Wall Street banks, it was amended and passed in the Senate. As the country’s financial system continued to deteriorate, several representatives changed their minds, and the House passed the legislation on October 3, 2008; President Bush signed it the same day.

It soon became apparent, however, that the government’s purchase of MBSs would not provide sufficient liquidity in time to avert the failure of several more banks. Paulson was therefore authorized to use up to $250 billion in TARP funds to purchase preferred stock in troubled financial institutions, making the federal government a part-owner of more than 200 banks by the end of the year. The Fed thereafter undertook a variety of extraordinary quantitative-easing (QE) measures, under several overlapping but differently named programs, which were designed to use money created by the Fed to inject liquidity into capital markets and thereby to stimulate economic growth. Similar interventions were undertaken by central banks in other countries. The Fed’s measures included the purchase of long-term U.S. Treasury bonds and MBSs for prime mortgage loans, loan facilities for holders of high-rated securities, and the purchase of MBSs and other debt held by Fannie Mae and Freddie Mac. By the time the QE programs were officially ended in 2014, the Fed had by such means pumped more than $4 trillion into the U.S. economy. Despite warnings from some economists that the creation of trillions of dollars of new money would lead to hyperinflation, the U.S. inflation rate remained below the Fed’s target rate of 2 percent through the end of 2014.

There is now general agreement that the measures taken by the Fed to protect the U.S. financial system and to spur economic growth helped to prevent a global economic catastrophe. In the United States, recovery from the worst effects of the Great Recession was also aided by the American Recovery and Reinvestment Act, a $787 billion stimulus and relief program proposed by the Barack Obama administration and adopted by Congress in February 2009. By the middle of that year, financial markets had begun to revive, and the economy had begun to grow after nearly two years of deep recession. In 2010 Congress adopted the Wall Street Reform and Consumer Protection Act (the Dodd-Frank Act), which instituted banking regulations to prevent another financial crisis and created a Consumer Financial Protection Bureau, which was charged with regulating, among other things, subprime mortgage loans and other forms of consumer credit. After 2017, however, many provisions of the Dodd-Frank Act were rolled back or effectively neutered by a Republican-controlled Congress and the Donald J. Trump administration, both of which were hostile to the law’s approach.