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There were two ancient civilisations in central and South America which the Spanish were particularly interested in. Both the Inca Empire of Peru and the Aztec Empire of Mexico held many riches. The Aztecs were the first great Native American civilisation with which the Spaniards came into contact.
The Senate is the branch of Congress in which each state is represented equally.
The ultimate goal of the United States in their intervention in Mexico in 1914 was to have a neighbor that is stable, that they will be able to influence and control to a certain level, and protect their economic and political interest.
The intervention of the United States was a very variable and very controversial. They did not stick with one side to support, but instead they were changing sides, usually supporting the person in power, except when the French intervened too.
Also, the USA openly threatened Mexico that it will use military power in order to protect its interests and its citizens that own businesses and property in the country.