Until Thursday afternoon, there had been a reassuring sense of restrained, tit-for-tat reciprocity in the trade skirmish between the United States and China.

But if this spirals into a bigger conflict between the world’s two biggest economies — something that seemed to become more likely Thursday evening with President Trump’s threat to add $100 billion more in tariffs — it’s worth keeping something in mind: In a trade war, the usual rules of commerce may not apply.

That is doubly true in a potential trade war with China, for several reasons.

Because the country exports far more in goods to the United States than it imports, China simply doesn’t have as much room to keep up with escalating American tariffs, especially given the Chinese government’s desire to cushion its citizens from higher prices for food staples.

Moreover, China has in the past proved willing to use a wide range of government powers to achieve commercial objectives — from campaigns against out-of-favor companies in state media to selective, stepped-up regulatory enforcement. And if things really get nasty, the United States and China are financially intertwined in ways that China could seek to exploit — though not without creating risks for a country holding $1.2 trillion in United States Treasury bonds.