The two statements that would work best as a thesis statement for the essay are:
- "The United States had valid reasons for going to war against Mexico in 1846."
- "The Mexican-American War was not justified."
These statements clearly take a definitive position on the justification of the war, allowing for an evaluation of arguments for and against it in the essay. The first statement supports the war's justification, while the second opposes it, presenting two contrasting viewpoints that can be explored.