OK, I feel like many people here don't know what this means.

The US sells bonds to people. These bonds are promises to pay them back more in a certain number of years.

The EU can't demand payment for all of that debt. What the EU CAN do, is selling the bonds it already owns, flooding the market. And not buying more themselves.

This would mean, that selling more bonds would become expensive for the US. Potentially they won't find enough buyers to fund their government.

Canada already threatened that (or started to do this, I don't remember) once already at the start of Trumps term. This might have been a factor for him backing of his invasion fantasies the first time.

Full disclaimer, I'm not a finance professional.

Thats the theory, but I'm guessing in real world applications it won't quite shake out like that.

As I understand it, existing bonds paid for past deficit spending by the US Government. New bonds are issued at regular intervals by the US for two reasons:

• to replace those bonds that have matured, but the debt still exists
• to generate new cash to pay for even more deficit spending.

Buyers for US bonds have two place to buy US bonds from:

• the US Government
• existing bond holders on the secondary market

Existing bonds that Europe holds have a fixed rate of return from when they were bought. So if Europe collectively started dumping US bonds in quantity, the value of the bonds would start to decline rapidly, but that wouldn't dry up the market for new bonds issued by the US Gov.

This would mean, that selling more bonds would become expensive for the US.

True!

The US Gov would have to raise the rate of return on newly issued bonds so that non-European investors interested in holding bonds would find the new US bonds more attractive because of the higher rate of return.

Potentially they won’t find enough buyers to fund their government.

Unlikely.

This will cost the US Government more to service the debt, but unless the entire world decided to stop buying US bonds, the US would not run out of money. As dystopian as the US government is right now, its still one of the best investments in this category. The USA has never once defaulted on its debt. The worst that has happened has been Quantitative Easing (aka "printing money") for a short time during the 2008 financial crisis, and once again in March 2020 during the worst (for financial markets) of COVID. Even then the US government reversed that with Quantitative Tightening (aka "shredding printed money") to raise the value of each remaining dollar in the system. Even with these historical actions, US government debt has been one of the worlds safest investments. Time will tell if that holds up or if trump blows that up too like all of our historically hard won alliances and soft power investments.

So existing bond holders worldwide would be hit hard. I'm guessing there wouldn't even be enough buyers for Europe to sell all their bonds at once, and they'd only be able to sell a fraction of them before the value of them plummeted. Europe could continue to sell what bonds they had, but they'd be losing significant amounts of money because they'd have to discount them to the point that the lower rate of return for those existing bonds was still a compelling investment for a bond buyer.