On Sunday, South Korean and U.S. Marines marched down an Incheon street as onlookers cheered, waving flags of both countries. Marines later hoisted the South Korean flag on a pier as a large outdoor screen flashed black-and-white images from the actual landing 75 years ago. If the Incheon landing symbolized the United States’ commitment to defend South Korea, then Incheon’s annual “We Remember” festivities embody the gratefulness many South Koreans still feel toward the Americans. But the alliance has been roiled in recent months as the Trump administration outraged South Korea by imposing heavy tariffs on its exports and by arresting hundreds of its citizens who were building a factory in Ellabell, Ga. This is not the first time that some South Koreans are questioning the dynamics of the relationship with Washington.