The Mexican-American War lasted from 1846-1848; the tension leading to the war started when the USA annexed Texas as a state in 1845.
Conscience Whigs was a faction of the Whig party which was harshly against slavery on moral grounds. To them, being against slavery was more important tha loyalty to the Whig party. They feared that the Mexican War would extend slavery to new territories, which was dangerous to the republic. So <em>they opposed it</em>.
The Transcedentalists was a group that started in New England before the Civil War. They had progressive views on abolition, education, reform and women's rights. They <em>opposed the Mexican War</em> as well, as they also considered it an imperialistic war to expand slavery. Since they believed in individualism and self-reliance, the transcedentalists were also against Americans imposing their views and politics on Mexico.
Lastly, the Southern Expansionists <em>defended</em> the war. In the 1840's, the expantionist mindset was spreading quickly; the expansionists believed they had the right to acquire more territories. The belief in the American "manifest destiny" was that they had to expand the "boundaries of freedom", pushing the American way to others. Aside from wanting to keep Britain away from the border by acquiring territories there, they also saw those territories as ungoverned and unprotected.
President James K. Polk used the expansionists justifications along with the border disputes to declare the war.
