Not really. Moreover, foreign assets more or less balance foreign debt. Most of that $31T is owed to Americans, or is intragovernment debt. I'm not saying it doesn't matter - because it does - but it's not a huge lever held by hostile foreigners that some people would have you believe.
In any case, it's not like anyone - foreign or not - can just "cash out" any of this debt, demand immediate payment for a bond's face value in gold doubloons. It's structured debt. A contract. No one can cash out a bond early; likewise, the US government can't "pay it back" early if the bond holder would rather collect the interest over the life of the bond. They can try to sell the bonds for market rates, but obviously (at least I hope it's obvious) if someone tried to dump $1T in bonds next Thursday, they'd have to find a buyer and they sure as hell wouldn't get $1T for them.
Foreign debt is not the problem.
Even if they were willing to take a loss on their bonds and dumped them on the market just for the sake of disruptively sowing chaos ... that's a suicide pact that probably hurts them more than us. Who buys the dump?
It's a fascinating circular thought experiment to contemplate these ideas, but the narrative that we're somehow beholden to and at the mercy of China or anyone else because of our national debt, just reflects a fundamental misunderstanding of what the debt is.
While we're speculating about what-ifs, I'll just throw out another thought experiment I like to post whenever this topic comes up:
Imagine you're an alien, watching tribes of primitive monkeyhumans on their backwards little planet orbiting a small yellow sun in the unfashionable end of the galaxy. One tribe, let's call them the nord'mericans, has spent a long time trading little green slips of paper and promises for more little green slips of paper to other tribes, in return for valuable, scarce, and/or nonrenewable resources. Like oil, or plastic lawn chairs. One day, the nord'mericans just say **** it, we're outta green paper and outta promises and we're gonna sit here looking across a couple of ocean moats, our continent is rich in natural resources, we can feed ourselves, we can make stuff (when we're bothered to get out of bed to do it) ... whaddaya gonna do?
Who do the aliens think won that exchange?
It's the biggest and most transparent con in the history of the planet. That we're the ones running the con is a little greasy, shameful, and disheartening ... but it's arguably better than being the ones getting conned.