To create a sense of cohesion between the introduction paragraph about Lincoln's opposition to the Mexican-American War and the subsequent paragraph that discusses historians rating him as America’s best president, an effective opening sentence might be:
"Lincoln's principled stance against the war not only demonstrated his commitment to moral integrity but also laid the groundwork for his later reputation as a transformative leader."
This sentence connects his views on the Mexican-American War to his legacy and how it is perceived by historians, maintaining the thematic focus on Lincoln's character and leadership.