A Mexican delegation arrived in Washington for meetings with U.S. officials to discuss immigration after President Trump threatened escalated tariffs on all Mexican imports unless it does more to stem the flow of migrants. WSJ’s Louise Radnofsky looks at what’s at stake for each side. Photo: Reuters President Trump said Tuesday he expects U.S. tariffs on Mexico to go into effect next week, despite ongoing negotiations to resolve a dispute over a surge in Central American migration to the U.S. Speaking at a press conference during a state visit to London, Mr. Trump said Mexico needs to step up and stem the flow of Central American migrants into the U.S. “I…