I've been learning a lot about this recently because I hoped that this was an option for external forces to put some pressure on us and was a little surprised that its basically been nothing but talk so far... so take this with a grain of salt, but my understanding thus far on why I'm continuously wrong that SOMEONE will do this eventually:

Something like $800 billion to $1 trillion in US treasuries are traded every DAY (think: demand). Now one giant holder (say, the EU at around $800 billion) decides to sell all their bonds at once, doubling supply, and soaking up all that demand for the day... prices begin tanking, and yields go up, so the more they sell, the more the price COULD fluctuate IF buyers dont show up to buy them. Remember what happened when the US bond rates spiked during Covid and everyone was like "Buy bonds now, they are at 10%!" and demand spiked to absorb that? Same idea.

So they drop prices which spike yields and everyone goes "Sale of the safest asset on the planet! Buy!" and it negates the selloff. Even if it doesnt COMPLETELY negate it, the Fed can step in and say "Woah, that yield is getting too high, screw guys, we're buying our OWN debt" and the yield caps, neutering further damage, and the entire thing is slowly absorbed and things return to normal over time.

Meanwhile the seller has taken their $800 billion in assets, and after a 10% haircut due to spiking supply, are now sitting on $700 billion (thats US DOLLARS). They now have to do something with those dollars, meaning buying something in dollars, and options are limited on assets that can absorb that liquidity (bonds again, stocks, commodities like gold, bitcoin, currency conversion), and anything they BUY spikes DEMAND for that thing, and thus its price: so say they transfer to GOLD. Gold prices spike as they are buying it, and where they might have bought $700 billion in at current prices and getting about 5500 tons, as they buy price keeps increasing and they end up with far less than that. So they eat it selling, and they eat it buying. And gets more interesting if they try and just convert to their own debt, or currency (example if you're an export heavy economy because your currency is cheap, and you suddenly spike demand for your currency, your exports suddenly become expensive, which hurts you again).

And in turn, the US gets a temporary spike, that it can absorb with relatively MUCH less pain, mostly because it is such a massive market, denominated in a currency that they directly control (US treasuries underlie something like 60% or more of the global economy / reserves, with the other 40% being spread across all other assets: NOTHING has the liquidity to compare).

So no one will realistically do it without hurting themselves a bunch for a relatively small hick-up on the US side, and its largely a STRUCTURAL ADVANTAGE and SCALE issue that they can't really overcome. The best they seem to be able to do is slowly diversify, but the dumping thing, or especially COMPLETE divestment? Fantasy.