GUANGZHOU, China — At his factory in southern China, hat supplier Jeffy Ma is trying to keep up with global demand as well as President Donald Trump’s seismic changes to international trade. His company, Ace Headwear, sews, embroiders and finishes golf visors, cycling headwear and baseball hats for brands such as Wilson and Fila, as well as for Major League Baseball teams. About 40% of the hats his 350 employees make are sold to the United States. “For us, the manufacturing profit is very low, so we cannot absorb so much,” Ma told NBC News last week at his factory...