Maybe the U.S. could call in all of the loans it has forgiven to other countries over the years?

Maybe. But IMHO it can be less expensive to forgive crippling obligations. Some historians argue that WWII was inevitable after the victors (particularly France) demanded punitive repayments at the end of WWI. I seem to recall that we may have discussed this some years back? In any case, had that obligation been forgiven, perhaps Germans would have focused on building cheap cars for the masses and jet passenger liners for holidays in Disneyland rather than goose-stepping into Bohemia. ;-) A useful task for the US today would be to convince the rest of the world that similar forgiveness would be good for everybody. Actually, it definitely would: getting the world economy chugging again is worth more than a lousy trillion or two. ;-)