That’s not really what “national debt” refers to… national debt is literal borrowing: “hey who wants to buy some bonds from my national government so we can invest in our economy?” Someone buys those bonds with the expectation of getting the invested amount + interest back.
What you’re talking about is most closely represented by “reparations” which is money owed by an aggressor to a victim state, and is only enforceable really by a stronger third party or by the aggressor losing the war.
As to why cities don’t take on debt the same way: they do take on millions of dollars of debt for infrastructure, but usually they’re loans from the federal government as opposed to bonds. The difference between city debt and national government debt is the national government controls its own monetary supply, meaning is defacto cannot default on its bonds. Cities can default on their loans, but typically the lender is the higher level government anyways so the repercussions tend to be political only. That’s why worrying about “the national debt clock” is typically not meaningful, but your city borrowing 300 million for a new highway definitely is.