America is quite small and everyone there wanted a big shining state that can take on the world (which it is now) First off they wanted a huge army. They also wanted to expand and gain new markets. Lastly they though of cultural superiority Aka Manifest destiny which was them invading Mexico and natives for land and fun fact USA cased the war so its not Mexico's fault for the Alamo TLDR: Wanting a bigger military more and bigger markets. And thinking their cultural is better than others
