SEOUL—South Korea is threatening to block a revised free-trade agreement with the U.S. unless its cars win an exemption from proposed American tariffs, putting at risk the only free-trade deal the Trump administration has successfully renegotiated.
The renewed economic tension between the two military allies comes four and a half months after they said they had agreed in principle on a revision to their six-year-old free-trade agreement. President Donald Trump, who described the original agreement as a “horror show,” said...