When other Group of 8 nations pressed Angela Merkel to support measures to stimulate euro-area economies at a summit over the weekend, it wasn’t because of a mere philosophical difference between the German chancellor and other G-8 leaders. Since 2008, the United States and Europe have put into practice two radically different theories of how indebted governments should respond to economic duress. America’s experience argues for economic stimulus in the short term — coupled with a concerted plan to bring down long-term liabilities.