I suspect a debt default by the federal government would result in a severe and negative reaction from the bond holders. Those bond holders are represented by nations that are or will become very hostile to the US if debt default becomes a possibility.
Do you think the US federal government would offer to ignore Taiwan’s pleas for defense as a partial recompense? You bet it would. And, I suspect that is not all. Which would become more vulnerable—NK or SK? SK would become vulnerable to the starving 19th century dictatorship in NK.
Treasury bills are in effect bonds. Right now, other major bond issuers are in trouble because so much liquidity has been snapped up with the purchase of T-bills.
With a default in US T-bills, international trade comes to a halt. But this doesn’t mean a complete cessation of US activities around the globe.
While Asian creditor nations would become upset, there is very little they could do about it directly. To a great extent, this is because the credit default would be an international crisis.
Taiwan might agree for a peaceful reunification with the mainland in such a crisis, because they might be able to get excellent terms. However, North Korea may be less inclined to invade South Korea than vice versa. They would be wise to hold their tongues, because China might be less supportive of North Korea than it is currently, and see the South Korean government as a less troublesome neighbor.
But back to the US, even though it would no longer be importing because of the default, it would still likely export food to the many nations we feed. Perhaps in exchange for some pro-rated debt.
The US military can “run on vapor” for quite a while, because many of its expenditures are for future activities a decade out. By contracting these and slashing pay, it can extend operations for a year or more with minimal funding.