The US raised the stakes in its drive for an aggressive response to the global financial crisis on Wednesday, calling for a tripling of the International Monetary Fund’s firepower and bigger fiscal stimulus measures worldwide. Tim Geithner, US Treasury secretary, called for the radical changes ahead of the Group of 20 finance ministers’ meeting in the UK this weekend. “Lots of things that did not seem realistic in the past are not just realistic but compelling,” he told reporters. “Forceful financial sector actions are critical to rebuild confidence, restore market functioning, get credit flowing again and bring stability to the...