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Where do countries / national governments borrow money from? 
It's more complicated than that. Governments raise money in a number of ways.  First, they tax economic activity within their borders and for connected companies and individuals.  Then, some governments have actual revenues from state-owned enterprises (licences, patents, courts, business revenues, and so on). Whatever shortage arises between state expenditure and this income is the deficit which is usually financed through debt. Government usually issues a bond (Wikipedia for a list of government bonds) of various types, some with extremely lengthy maturation dates.  These bonds can be purchased both locally and by foreign investment funds.  The nature of who buys is important. From the Wikipedia link you'll see that most government debt is very highly rated based on the ability of the state to simply raise taxes in order to fund redemption.  Pension funds are legally bound to only invest in highly-rated investment classes and the bulk of bonds may be purchased to support local pensioners. A state that defaults on debt will first hit its own most vulnerable citizens.  In addition, the fall-out will result in a savage cut in ratings.  Countries like Argentina and Zimbabwe, which have both refused to repay their debts even to the IMF, are currently unable to raise investment at all.  This has a tremendous impact on local economic development. So, default is out of the question without severe penalties. The second part of your question is about paying down the debt.  As debts increase, more and more of the revenue that a country does earn is spent on servicing debt repayments.  Sometimes bonds are issued merely to refinance old debt.  A country that spends too much on refinancing debt is no different from an individual.  Less and less money is available to do other things. In conclusion: governments can neither default nor binge-borrow unless they wish to severely limit economic opportunities for their citizens.