What were the causes of the Mexican-American War? How did the war compare with the War of 1812 in terms of results and popular feelings? What groups or individuals supported the war, and which were against it? Give at least two examples of pro-war groups or individuals and at least two examples of anti-war groups or individuals and explain their actions and why they felt as they did.

Both the War of 1812 and the Mexican-American War were not supported by Congressmen in the northeast. After 1836, when roughly 10,000 Americans who lived in the northern section of the recently independent Mexico, gained their independence and founded the Republic of Texas, they applied for statehood with the US twice, but Congress denied the state admission into the Union. Northerners did not want another slave state added to the Union.
James Polk pushed for the state to be annexed due to the push for Manifest Destiny.
James Madison dragged the US into the war with Britain in 1812. New England's economy had already been decimated by Jefferson's Embargo Act of 1807, and they did not want to lose more trade with England nor risk losing or paying for an expensive war. A large group of Federalists met in Hartford in 1812, and New England almost seceded from the Union, but Madison won the war before they did.
The War of 1812 frightened northern merchants and the Mexican-American War frightened northerners who feared slavery would be allowed throughout the entire US, which would change political legislation such as import tariffs, which Southern states' congressmen did not support.