The U.S. government seized control of American International Group Inc. -- one of the world's biggest insurers -- in an $85 billion deal that signaled the intensity of its concerns about the danger a collapse could pose to the financial system.

The step marks a dramatic turnabout for the federal government, which had been strongly resisting overtures from AIG for an emergency loan or some intervention that would prevent the insurer from falling into bankruptcy. Just last weekend, the government essentially pulled the plug...