It is a security that it SOLD.
No one at the US Treasury went hat in hand to the Chinese or "the bankers" or anyone else and said "Please PLEASE lend us 12 Trillion dollars!"
The overwhelming amount of debt the US Treasury has outstanding is in the form of commonly issued bonds - bonds that are regularly AUCTIONED and no one, NOT ONE SINGLE PERSON OR ENTITY, is obligated to participate in the auctions or buy the bonds.
"we borrowed it from banks and other governments."
Wrong.
Banks and other governments PURCHASED US Treasury securities because they are seen as just about the safest, most liquid security in the world. They pay their interest payments on time and they are redeemed at par on schedule, completely predictably.
Here's a question for you;
China (or whoever) holds X amount of 30 year US bonds with 25 years to maturity. Now all of the sudden they decide they don't want them anymore, for whatever reason. Can they demand their money back from the Treasury?
"I'm beginning to wonder if Krugman himself understands debt. . . . ."
And I have wondered for quite a while if you understand the bond market at all, how bonds work, how they are issued, the difference between yield and coupon, what a callable bond is, what a putable bond is, par, discount, premium, yield to maturity, yield to call, current yield, etc.
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