To begin with, the US was never "neutral". Roosevelt was planning to get involved (he helped set it up) in WW2 once he realized that his New Deal programs were not successful in bailing the country out of the Great Depression.
Roosevelt provoked conflicts between Poland and Germany to get the war started in Europe, but couldn't get Congress to support a war on the other side of the world that the US didn't have a stake in.
Roosevelt then provoked Japan into attacking so as to "bring the war to America" as it were. This guaranteed the Congressional thumbs up to go to Europe.
But Japan wasn't trying to conquer China. That feud was the result of a disagreement over which country had control of Manchuria and the Soviets got involved somehow (not sure of the exact details since I'm not versed on that aspect of it).
Roosevelt threw support at China and began setting up blockades around the Pacific theater to prevent Japan from acquiring the oil that they needed. This, in turn, laid down the breadcrumbs leading Japan to Pearl Harbor.
The well-known "Rape of Nanking" never happened. It was Chinese propaganda meant to demonize Japan just as the "Holocaust" was meant to demonize Germany.