They borrow, usually from the Chinese who are the only ones who are willing to lend the U.S. money. The day the Chinese refuse, is the day our economy freezes. Because our government has over a million people on the payroll, if Washington D.C. is delayed in paying people's salary, it would devastate the economy. Other governments are much smaller so if they fail to pay their employees for several months it doesn't impact their economy as much. The problem starts when the interest payments increase to the point that there isn't enough money coming in to even pay the interest. In Greece interest + salaries were greater than the income from taxes so the government defaulted and Greece had a large government so that is why it impacted the economy so greatly.