Why did the U. S. oppose Japan's imperial aspirations in Asia?

Because they had war

While the United States was still struggling to emerge from the Great Depression at the end of the 1930s, and would do so partly because of the war, Japan had emerged from its own period of depression, which had begun in 1926, by the mid-1930s. Many of the young soldiers mobilized into the Japanese army by the early 1930s came from the rural areas, where the effects of the depression were devastating and poverty was widespread. Their commitment to the military effort to expand Japanese territory to achieve economic security can be understood partly in these terms. The depression ended in the mid-1930s in Japan partly because of government deficits used to expand greatly both heavy industry and the military.