President Trump announced a 30% tariff rate for the European Union and Mexico effective Aug. 1, challenging the financial markets' resilience to tariff threats.
Despite warnings, investors have assumed previous U-turns from the administration and minimal impact on markets.
The EU had been negotiating a tentative deal with the US to avoid higher tariffs, but optimism was punctured by Trump's letter.
Financial markets have struggled to price in the on-again, off-again tariff campaign by Trump, with mixed reactions in response to the recent trade offensive.