What role did Abraham Lincoln play in the Mexican-American War?

B. As a Congressman, he opposed it.

Explanation:

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Digital History. Annotation: Elected as a Whig to Congress in 1846, Abraham Lincoln gained notoriety when he lashed out against the Mexican War, calling it immoral, proslavery, and a threat to the nation's republican values.

Early on in Lincoln's political career, as a freshman Whig congressman from Illinois, he condemned the 1846 U.S. invasion of Mexico, bucking the prevailing patriotic tide and accusing President James K. Polk of promoting a falsehood to justify war.

Explanation:

When the war began, it was my opinion that all those who, because of knowing too little, or because of knowing too much, could not conscientiously approve the conduct of the President, in the beginning of it, should, nevertheless, as good citizens and patriots, remain silent on that point

In short the answer is B