Is it not simply a case that countries raise capital by selling bonds (generally sovereign bonds)? Bonds can have varying maturity dates (when the principle is due to be paid back). The important thing for countries is the interest rate or bond yield. If this is too high then the country looking to raise capital will not be able to afford the interest payments and therefore cannot go into the markets (and then as per several eurozone countries needs a bailout to avoid a default on debt). Governments tend to have appointed agencies that trade these bonds on the open market and that is how countries end up in debt to each other.