A good point. Thank you.

The violence, once enacted, is an objective fact.

The threat of violence is still there only due to the collective subjective agreement that countries exist.

The IRS would have no functional power if Washington, D.C. and other major US cities were destroyed in a nuclear exchange.

I could imagine many citizens still choosing to pay taxes of some sort and/or respect the value of physical dollars in such a scenario, but it would likely not be due to fear of the federal government. Possibly fear of local police, though.