The two statements that would work best as a thesis statement for your essay are:
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"The Mexican-American War (1846-1848) was a conflict between the United States and Mexico that resulted in the U.S. acquisition of large parts of what is now the American Southwest. Evaluating the arguments for and against the war provides a nuanced understanding of its justifications and criticisms."
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"The United States had valid reasons for going to war against Mexico in 1846."
These statements provide a clear stance on the war and the parameters for analyzing the arguments surrounding it, making them suitable thesis statements for your essay.