At the start of 1940 Germany and the Third Reich had already begun its spread and domination of Poland. While the United States was not yet a major player on the world stage, we still had a negative outlook on the expansion of Germany and the beginning of World War II. On September 27, 1940 Japan signed a treaty with Germany and Italy to become war allies. This allied Japan against the US as the US was also allies with England, France, and other direct enemies of the Nazi movement.
One of the other significant reasons the US was opposed to their actions was the United States, while not having much of a hand in Europe, did control islands in the Pacific such as Guam and the Philippines. Japan's expansion began with their taking parts of China, but Japan's interests started to spread toward the US-controlled islands. To slow their movement and momentum the US imposed economic sanctions against Japan, essentially warning them that if they continued to ally themselves with the Germans and Italians as well as attempt to dominate the US-controlled islands they would lose the US and its allies' trade and business. This led Japan to run very low on materials and goods such as oil, and rather than rethink their position they pushed the offensive by attacking military bases belonging to the US and British in China and the US-owned islands. This led to Japan bombing Pearl Harbor which finally gave enough reason for the US to commit troops and supplies to World War II and began official involvement in the war.
In summary, there were two prominent reasons the US was opposed to Japanese expansion. The first was that Japanese expansion threatened US-owned islands and military bases in China. The second was that a Japanese success would also mean a success for Germany and other enemies of the US's allies like France and England. Hope that helps!