We've all heard that "China owns the majority of the U.S. debt" and they will, one day, somehow, own the entire country. It's not that simple.
Japan is America's largest debt holder, holding roughly 18.67 percent, or $1.27 trillion, of the country's debt. China owns 15.88 percent, or nearly 1.1 trillion.
The reason why these countries and others buy U.S. debt is because it's safe. Debt is sold in the form of Treasury notes, bills and bonds, which are historically a safe bet for long-term appreciation. Other countries' demand for our Treasurys keeps America's interest rates low, which increases federal spending and helps economic growth.
If China (or Japan) decided to call in its Treasurys, the dollar would collapse, and it would trigger a global economic depression worse than the 2008 financial crash. There are no winners in that scenario, especially not China, which relies on U.S. Treasurys to boost its economy and establish its creditworthiness.
Additionally, if China sold off its Treasurys, the cost of Chinese products would cost drastically more to produce, and China would effectively be crippling itself on purpose.