President Trump on Monday said the U.S will impose 25% tariffs on goods from South Korea and Japan by Aug. 1, while also threatening to hit a dozen other nations with steep import duties by next month.

Trump uses tariffs as a key tool to force changes to U.S. trade relationships, but his on-again-off-again approach has roiled markets and stoked uncertainty.

After months of threats, President Trump is abruptly re-escalating the trade war.For businesses craving certainty, who've had a period of relative calm, the move could once again upset that balance.It also injects a new risk into financial markets that have largely already moved on to a post-tariff world.