A lack of trust and the whack-a-mole nature of stamping out the chemicals used to make the drug help explain why the war against fentanyl has been so intractable.

China's Commerce Ministry on Tuesday vowed to retaliate against fresh U.S. import tariffs set to take effect later in the day, reiterating its stance that the Trump administration was trying to "shift the blame" and "bully" Beijing over fentanyl flows.