How US,UK,Japan,.. and all the powerful economies have external debt nearly equal or more than their GDP?

There are a number of factors that explain why the developed nations of the West have higher debt-to-GDP ratios than other nations: extensive social programs financed through deficit spending, waging wars while cutting taxes in the case of the U.S., low interest rates that wealthy nations can pay on borrowed funds, which encourages borrowing and actually gives a financial incentive to accrue debt if you can grow your economy at a higher rate that interest rates, lack of fiscal discipline, trade deficits, etc. That said, most sovereign debt is held by domestic bondholders, which is not a net liability to a nation, as it represents a claim by one citizen against another citizen of the same nation. However, debt held by foreigners is a national liability, as it represents a claim on a nation's capital by another nation. So, to answer your questions, most of the debt is held by domestic investors and institutions and the remainder is held by foreign investors and entities. But, much of the debt hold by foreigners is actually held among the wealthy nations themselves. For example, Japan holds a relatively large amount of U.S. debt (they are both wealthy nations). The reason that China and nations of the sort have such low debt ratios is the fact that they have large trade surpluses. Finally, the UK is not bankrupt. Wealthy nations have large lines of credit so they simply refinance their debt by issuing new debt when the old debt comes to maturity. I'm not saying that high debt is not a problem, as it may cause investors to become nervous and ask higher interest rates which can make refinancing and financing so expensive it leads to default (that is what is happening in Greece). However, take the alarmism over nation debt with a grain of salt.