DUBAI, July 2 -- For American motorists, the US-Iran ceasefire delivered something straightforward: cheaper fuel for the first time in months, as Brent crude posted its worst quarterly decline since 2008, shedding roughly $45 to settle around $72 a barrel. The less obvious consequence, and the one that trade economists are watching far more carefully, is what that cheaper oil hands Donald Trump next.

For two years, a consumer inflation rate that peaked above five percent kept the White House's trade ambitions pinned. Tariffs on imported goods tend to push retail prices higher, and the political cost of adding to household budget pressure had outweighed the leverage that duties provide. The arithmetic has now changed. With Brent near $72 ...