Answer: in the 2nd part of the 19th century USA were convinced of their racial superiority (also thanks to social Darwinism, biologization of social sciences and biological determinism that existed in Europe but also in the South America) and felt their obligation to reform the "inferior races" of the region. So Washington bureaucrats attempted to reshape countries such as Cuba into model versions of their own republic. That is what happened in times of Theodor Roosevelt´s bureaucracy. But before that "conquest" of the South was something which was an affair of adventurers and entrepreneurs with frequently economic interests (William Wheelwright in Chile, Stephens in Central America).
Explanation: Presidents like Thomas Jefferson and John Adams viewed Spanish possessions, including Florida, today´s Texas, and Cuba, as regions that should and would be incorporated into the United
States. Racial aspect was always present ...many people in the USA saw themselves as part of a superior Anglo-Saxon race.