While the financial sector bailout of 2008 and 2009 may have benefited some undeserving bankers, it was necessary to ensure the economy didn't fall into a depression, says former Treasury Secretary Tim Geithner.

"What one has to do in a panic is the opposite of what seems fair and just," he writes in The Wall Street Journal.

"In a financial crisis, the natural instinct is to let creditors suffer losses, let firms fail, and protect taxpayers from any risk of loss. But in a financial panic, a strategy based on those instincts will lead to depression-level unemployment."

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So the government and Federal Reserve have to step in with "a massive injection of cash" to keep the financial system from collapsing, says Geithner, now president of private equity firm Warburg Pincus.

He maintains that the bailout turned out well.

"Because two presidents were willing to put politics aside and deploy a massive and creative rescue, we prevented economic catastrophe and got the economy growing again in about six months," Geithner explains.

He acknowledges that problems such as unemployment and income inequality still exist. "But we did do the essential thing, which was to prevent another Great Depression."

Edward Luce of the Financial Times gives a mixed review of Geithner's new book, "Stress Test: Reflections on Financial Crises."

"Will history see Geithner as a great Treasury secretary?" Luce asks. "That is uncertain. He was certainly effective. But too much of this otherwise self-deprecating memoir is self-defense."

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