Won't countries be in trouble if all sovereign debtors demand payment?
The principal has to be repaid eventually, but with majority of formal debt instruments such as government bonds the creditor can’t just demand repayment whenever he  wants. Most debt instruments specify the date, and if not date, then conditions when and how the principal is being paid back.


For example, a standard 10 year bond will pay its principal in 10 years not a day earlier, unless you convince the debtor to voluntarily agree to pay it sooner).


The above works for sovereign countries too. A 10 year government bond will have on it printed that principal is paid back exactly after 10 years. 


Now this is outside economics, but technically sovereign nations are in state of anarchy. So I can imagine one country could just decide to say to another — pay now or we will invade you/nuke you etc! But aside some military intervention, you can't legally force the country to pay.

In fact any government can, in principle, default on its debts anytime it wants. The only constraint on sovereign default is that if you constantly default on your debt, nobody will want to lend you money in the future, or if only at punitively high interest. Hence on sovereign level, governments as debtors have much better bargaining power than just individuals.

Government can be forced to pay its debt only through military action, or in some cases by diplomatic pressure (for example EU could threaten Greece by revoking its euro-zone or EU membership, US could threaten countries by economic sanctions etc.). Aside from that, there is nothing other country can do to get its money back if debtor nation says no.


Based on 1, and 2 you probably already know the answer. No that’s not possible. China could patiently wait for bonds to expire and then not buy new US bonds, but instead buy Australian or New Zealand bonds. Of course, since US government issues a lot of debt, bonds expire probably every month and are replaced by new ones, so China could start to refuse to buy new US bonds sooner than later.


Also, usually when people want to get rid of a debt, they won’t wait for maturity and for repayment of the principal. They will take the bond to bond market and sell it at market price to get money now. That creates a trouble for country because now it has to set lower price or higher interest on its newly issued bonds due to increased supply, but whether this becomes a problem depends on plethora of factors unrelated to the original debtor and creditor countries, such as how big is the demand for the debtors bonds etc.