How did the U.S. expand itself imperially in the late 19th and early 20th centuries and why interested in obtaining control over foreign territories?...

The motivations of American imperialism were complex, but essentially
grew out of the lack of space for "Manifest Destiny"-type expansion once the West
Coast was reached, ethnocentrism and the desire to share the "superior" institutions
of American-style democracy.  Economic pressures were also involved, of course, and the same
imperialist desires which motivated Great Britain, Rome and all previous
empires.

When America's Manifest Destiny ran right to the
edge of the Pacific Ocean, many economic and political interests in the US came to believe that
the country's future lay in expansion on the Pacific Rim.  Hawaii and Alaska (including the
Aleutian Islands) became American territories, American trading interests in China expanded
and...