Although this depends slightly on what period you're referring to during this time, the best answer would be that "Immigrants" made up the largest percentage of workers in American industry during the late nineteenth century, since the economy had shifted towards industrial output as opposed to agrarian output.
Answer: The amount of resources and space for industry, and advancements, as well as the technology from Europe that the U.S. had access to, made the U.S. the dominant power in the Western Hemisphere.
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