China put a small tariff on American oil, because they wanted to save face, and continue to posture as a mighty power, that can go toe to toe trading blows with the USA economically. If investors and creditors see China as what it is - a mafia operation conducting the largest financial fraud in Human history, the flow of real money could dry up, and their house of cards would implode.
Oil is is a commodity product (like soybeans). Commodities are the class of imports for which they can most easily find substitute suppliers. Most American imports are things they really need for their economy, because everything else has long been blocked out by their many unfair trade practices.
So they “retaliated” with tariffs that would total about $5 billion per year, if volume stayed the same. President Trump responded with $27 billion per year on them, that afternoon. Now they graciously say they don’t plan to retaliate again “at this time”. They have very little left they could do to the USA with tariffs, and major room for the USA to put more painful tariffs on them.
The fight is over, it will just be a well deserved beating from this point on.