For the most part, this overseas empire was a culmination of the expansionism that the US had engaged in at home. It was caused by many of the same factors as the earlier expansion.
The earlier expansion within North America was largely driven by the idea of Manifest Destiny. Americans believed that they were destined by God to expand their territory. They felt that God had chosen them because they had a superior culture, a superior governmental system, and a superior religion. This made them more deserving than the peoples they conquered. Of course, the US also wanted to expand for economic reasons and to increase the nation’s power.
The same could be said for the overseas expansion. The US felt that it deserved to take places such as Hawaii and the Philippines because its people were superior to the natives of those islands. They felt they were “taking up the white man’s burden” by civilizing these people. They also wanted to become more powerful militarily and economically. All of these were the same factors that drove the expansionism within North America.
As the previous educator has noted, Manifest Destiny, as well as pseudo-intellectual ideas about race, influenced the United States' imperial ventures in North America and in the Pacific.
However, there were a couple of other factors, including the impact of the Monroe Doctrine and the end of slavery in 1863.
The Monroe Doctrine, instituted in 1823 by President James Monroe, opposed further European colonialism in the New World. By this time, both France and Spain had lost many of its acquisitions in the New World. The United States promised not to interfere with those that currently existed, but would not abide European countries that sought new territory. This doctrine established United States sovereignty and influence in the New World, as well as its capability as a military power, in that it had the force to stave off interference and imperial ambitions from Europe. According to philosopher Noam Chomsky, this allowed the United States to set itself up as a hegemonic presence with the right to interfere in the affairs of other nations throughout North and South America. This assumed right to interfere, coupled with notions of superiority and an agreement from Europe not to pursue additional territories, paved the way for American Imperialism.
The historian Sven Beckert has argued that the end of slavery in the United States led to both America and Europe's desires to colonize other nations. Textile mills in both the Northern United States and Great Britain, for example, depended on mass-produced cotton from the South. There was still a demand for cheap, mass-produced cotton, in addition to other resources that become necessary with the expansion of industrialism, such as minerals.