CHINA - China holds more than a trillion dollars in United States debt, and an escalating trade war could tempt it to wield that debt in a way that has long been unthinkable. It is often called the nuclear option. Critics who dismiss the possibility of China trying to upend the United States Treasury market say that China’s own economy is too fragile to risk doing anything that would cause instability. Supporters of the Trump administration’s tough stance on tariffs with China take actions like that as a sign that the United States holds the negotiating leverage. And it remains an open question whether China could inflict real damage by selling Treasuries. But brinkmanship does not breed rational thought. The escalation of hostilities, even economic ones, raises both stakes and tempers alike, which is a dangerous combination. And in this case, there is no proving ground. There is no predictable math, no scale model.