The two statements that would work best as a thesis statement for your essay are:
A. 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.
B. The United States had valid reasons for going to war against Mexico in 1846.
Statement A sets up a balanced evaluation of both sides and indicates that you will analyze the complexity of the situation, while statement B clearly states a position in favor of the war's justification. Depending on which argument you develop, either of these could work effectively as the thesis for your essay.