To get a glimpse of the dynamics behind the escalating trade spat between the U.S. and China, look 1,000 miles south of China’s border to the Vietnamese port of Vung Tau.

On the coast near Ho Chi Minh City, dozens of factories follow a simple business model: Import steel from China, galvanize it, strengthen it and then export it—often to the U.S. at prices that undercut American producers.

In less than two decades, several Vietnamese companies have used this blueprint to make their country one of the fastest-growing suppliers of steel to the U.S. Vietnam now accounts for about 2% of U.S. imports of the metal.

The companies, as well as Hanoi and Beijing, argue that they are playing by the rules of global trade, buying the cheapest raw materials, turning them into more-sophisticated products and selling them to the highest bidders.

U.S. trade officials, however, say the companies and their Chinese suppliers are guilty of transshipping—routing goods through another country to illicitly disguise their origin.