How did the united states become a potential threat to spanish control of its colonies

The United States became a big threat to the Spanish colonies with its grow in size and power, both in militaristic and economic aspect, but also very important factor was that the United States with their growth in size managed to get to the Spanish colonies, thus share a land border with them. With the countries being separated just by an imaginary line, it was much more dangerous for the Spanish because they were not as strong as they used to be, and they had a growing power knocking on their door.