By Bethany McLean
The opinions expressed are her own.

On Monday morning, MF Global, the global brokerage for commodities and derivatives, filed for bankruptcy.  The firm’s roots go back over two centuries,  but in less than two years under CEO Jon Corzine, whose stellar resume includes serving as the chairman of Goldman Sachs, as New Jersey’s U.S. Senator, and as New Jersey’s governor, MF Global collapsed, after buying an enormous amount of European sovereign debt. The instant wisdom is that he made a big bet as part of his plan to transform MF Global into a firm like Goldman Sachs, which executes trades on behalf of its clients, and also puts its own money at stake. Although the size of the wager has received a great deal of scrutiny, the accounting and the disclosure surrounding it have not–and may have played a role in the firm’s demise.

In the 24 hours since the filing, more ugly questions have piled up, with the New York Times reporting that hundreds of millions of dollars of customer money have gone missing, and the AP saying that a federal official says that MF Global has admitted to using clients’ money as its problems mounted. Whether this was intentional or sloppy remains to be seen; MF Global didn’t respond to a request for comment by press time.

At the root of MF Global’s current predicament was a simple problem:  the profits in its core business had declined rapidly.  That core business was straightforward, even pedestrian; what the firm calls in filings a “significant portion” of total revenue came from the interest it generated by investing the cash clients had in their accounts in higher yielding assets and capturing the spread between that return and what was paid out to clients. As interest rates declined sharply in recent years, so did MF Global’s net interest income, from $1.8 billion in its fiscal 2007 second quarter to just $113 million four years later. MF Global’s stock, which sold for over $30 a share in late 2007, couldn’t climb above $10 by 2009.

Enter Corzine in the spring of 2010, who had just lost his job as New Jersey’s governor to Chris Christie. He was brought in by his old pal and former Goldman partner Chris Flowers, whose firm had invested in MF Global. Fairly quickly, Corzine accumulated a massive net long sovereign debt position that eventually totaled $6.3 billion, or five times the company’s tangible common equity as of the end of its fiscal second quarter. I’m told Corzine’s move was highly controversial within the firm.  But no one overruled him, maybe because after all, he was Jon Corzine. In a mark of just how much Corzine mattered to the market, in early August, MF Global filed a preliminary prospectus for a bond deal, in which the firm promised to pay investors an extra 1% if Corzine was appointed to a “federal position by the President of the United States” and left MF Global.

Buying European sovereign debt may not have been just a bet that the bonds of Italy, Spain, Belgium, Portugal and Ireland would prove attractive. An additional allure may have been the way MF Global paid for the purchases, and thereby, the way the accounting worked. MF Global financed these purchases, as its filings note, using something called “repo-to-maturity.” That means the bonds themselves were used as the collateral for a loan, and MF Global earned the spread between the rate on the bonds, and the rate it paid its repo counterparty, presumably another Wall Street firm. The bonds matured on the same day the financing did.