Theoretically, if the US went to war with a country who held its debts, does that affect the debt?
US Constitution Amendment 14 Section 4 states:
"The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any state shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void."
The important part is that the debt shall not be questioned. The amendment goes on to further state that rebellions and insurrections wouldn't add to the debt. I don't know if you can say that means parties to war would add to our debt if we took them over, but again, the important thing is that the validity of the debt...shall not be questioned.