That’s… not really accurate. Some factions in the US flirted with taking over more or even all of Mexico for the grand total of like, a few months, and even then it never would have passed Congress. Additionally “the US elite” is a flawed and inaccurate understanding of mid-1800s politics as even the most basic of historical understanding will tell you. It’s mostly regional politics and economic constituencies, not class struggle. You’re just projecting modern thought and dynamics onto the past.