Which prominent figure opposed the mexican-american war?

Henry Clay Sr.

Explanation:

Henry Clay Sr. was an attorney and politician from Kentucky. The state of Kentucky was represented by him in both the Senate and the House of Representatives of the United States. At the time when the then U.S. President John Tyler made an annexation agreement with Texas clay announced opposition to this annexation. Clay argued that the nation does not require any annexation due to the very fact that it would bring tensions and what the country need is  "union, peace, and patience".

one of the main reasons was definitely slavery. back in 1861, republican abraham lincoln was elected and ended up being a major influence in the slavery business. slavery greatly affected the texas economy because back in 1845 during the annexation of texas, slaves became a big part of the texas population. many texans that were in favor of the secession influenced groups through party ties.