Most of the debt Italy has taken has been taken under Italian laws. The Italian government can rewrite those laws at will. Sovereigns define their own laws as t

Most of the debt Italy has taken has been taken under Italian laws. The Italian government can rewrite those laws at will. Sovereigns define their own laws as they see fit - that is the meaning of being sovereign. Sovereigns entering obligations under laws not their own is a totally different thing though, and cannot be changed this simple though. As to how the US would react in such a hypothetical scenario, they'd probably send diplomats to Italy to try to bend some arms and influence the terms to be better for them, but it is the Italian government that is the bookkeeper for deals made under their own laws.