LONDON — As President Trump intermittently escalates and moderates his trade war with China, his conflicting signals reflect a reality that limits his actions: He can try to sever the deeply intertwined American commercial relationship with China, or he can prod economic growth to assuage the fears of investors around the planet.

But he cannot do both at the same time.

Mr. Trump need not rely on the testimonials of economists to deduce this. He can disregard the admonitions of news outlets he derides as fake news. He can simply consult the one source whose verdicts he tends to celebrate: the stock market.

Among those who control money, portents of further trade hostilities between the United States and China, the two largest economies on earth, have proved an impetus to sell with abandon while amplifying talk of recession. Intimations of a deal avoiding further animosity reverberate as a clarion call to buy, sending share prices higher while easing worries about a potential global economic downturn.

Mr. Trump often appears caught between competing impulses that pull markets — and his China policy — in opposite directions.