Too bad U.S. businesses don’t have a mantra comparable to Britain’s famous World War II “Keep Calm and Carry On” slogan to get them through the current tariff turmoil.

Despite such a rollercoaster ride so far, market investors, in response to most developments, seem to have adopted the U.K.'s mantra as it prepared for war in 1939: Keep calm and carry on.

Polish Prime Minister Donald Tusk on Tuesday told European leaders to “KEEP CALM AND CARRY ON” amid tensions in the U.S.-Ukraine relationship “To all the European leaders: KEEP CALM AND ...