The USA can simply refuse to pay up. However, that would instantly crater their credit rating. Going from AAA straight to D would make future lending very expensive.
Countries basically can't work without borrowing money. That's what these treasuries are. The EU holds a crapton of US government debt; 10 trillion dollars with all types of assets combined. Liquidating them sends a clear message that US debt is no longer worth holding onto. That leaves us with two scenarios:
- The USA pay off their debt, instantly draining trillions of dollars out of their coffers. They will have to issue new bonds to pay for this; issuing this many bonds for this reason will be seen as a sign of economic weakness and lack of reliability. Even with their AAA rating, the USA wouldn't get great deals for those bonds and their main customer has just called it quits. The credit rating may be reduced because of this.
- The USA refuse to pay. That's equivalent to being unable to pay; their credit rating instantly goes to hell. The only reason people love to lend money to countries is because those debts are stable and reliable. If a government fails to pay their debts back for any reason, especially at that scale, that government instantly loses all credibility as a debtor. That means future US debt won't be issued at something like 1% interest but more like 10-20% interest. And that interest will have to be paid, reliably, if they want any chance of climbing out of that hole.
Either way, it's economic warfare, and not the border skirmish kind. This could very well completely fuck the US economy for decades.
Would the USA retaliate? Sure. But this is already in a scenario where the USA pose a direct military threat to the EU. It'd simply add another layer to the war.