The ships laden with grain recently leaving the ports of Ukraine might be taken to suggest that those who doubted it was possible for Ukraine and Russia to negotiate had been wrong. But with the exception of a brief meeting between U.N. Secretary General António Guterres, Turkish President Recep Tayyip Erdogan, and Ukrainian President Volodymr Zelensky, there have been no recent diplomatic attempts to negotiate an end to the war. Instead, the U.S. has consistently discouraged negotiations. U.K. Prime Minister Boris Johnson told Zelensky not to negotiate with Russia. In his dying days as prime minister, he repeated that call...