If it spends enough to trigger the debt bomb literally pounding sand, that could do it. It isn't Iran that is the danger though. The US could just walk away any time and be fine.

I'm sure people said the same about the USSR invading Afghanistan.

The US right now cannot keep its bridges from collapsing. It cannot keep its children safe from men with guns. It cannot keep its citizens fed or housed. It is failing to provide adequate healthcare for a majority of its population, it cannot even keep its children vaccinated against measles. Our science agencies are being run by crackpots. Our mass media is being combined under one single owner.

This doesn't even consider the impending existential challenges of climate change.

And this nation, instead of fixing its crumbling domestic infrastructure - educational systems, health care systems, or anything that would benefit the citizens of the US - has chosen to launch an attack against a foreign nation that has already cost 10s of billions of dollars and will likely cost vastly more.

All the political and economic capitol that is required to maintain and improve stable conditions is instead being poured into murder in a desert thousands of miles from home.

I'd argue that Vietnam and Al-Qaida/Afghanistan/Iraq where much lesser "risks" than Iran, and those still left lasting scars on US society, self-image and standing.

Well, why don't you open Bloomberg or the Financial Times to understand why. The damage being caused is potentially civilization-ending. At the very least, this is already going to be very expensive for everyone for many years.