they wanted Texas back because at the time Texas was NOT ruled by anyone but they wanted to be annexed into the united states.America was like "hey if we let them join we get more land and expand as a country." and so they did, but when Mexico heard about this they were like "imma bout to start swinging" turns out they lost not only Texas but soldiers and money. America later buys the rest of the northern territories such as california,Arizona and new Mexico.