Final answer:
Japan's upset with the U.S. before World War II was fueled by Western influence, unfair post-WWI treatment, and immigration laws, further exacerbated by U.S. economic sanctions and military posturing that threatened Japan's expansionist goals.
Step-by-step explanation:
Japan began to get upset with the U.S. in the period leading up to World War II due to a combination of factors. These included perceptions of Western influence and unfair treatment post-World War I, as well as resentment over the U.S. laws prohibiting Japanese immigration.
Economic and military expansion played a significant role in escalating tensions; Japan's ambition to assert dominance in the East clashed with U.S. economic sanctions, including an embargo on oil and gas, imposed to curb Japanese aggression. These actions were viewed by Japan as an obstacle to their plans in Southeast Asia and the Pacific.
Moreover, the transfer of the U.S. Navy's Pacific Fleet to Pearl Harbor was perceived as a direct threat to Japan's interests. The deteriorating relationship between the two nations failed to improve despite diplomatic efforts, leading Japan to plan and execute the surprise attack on Pearl Harbor, which propelled the U.S. into World War II.