Texas is finally annexed to the Union just as John Tyler leaves office and James Polk takes office. President Polk realized there was a good deal of opposition to the Mexican War: potential opposition in the Whig party and in the North because of the possible expansion. However, President Polk gains military victories and the war ended in 1848 with the annexation of the Mexican Territories to the Union. (Hume, Video Lecture 4).
With the deaths of Andrew Jackson (1845), John Calhoun (1850), Henery Clay (1852), Bennett (1858), Daniel Webster (1852), and Martin Van Buren (1862), among others, many of the reform issues were being faced by new political leaders; facing these issues first as Whigs and Democrats and later as Northerners and Southerners. All of these new leaders appear on the scene about the time of the Wilmont Proviso Crisis; the first great sectional crisis provided by the Mexican War. The Wilmont Proviso Crisis and in a sense the Pandora's Box that this crisis opened, politically it is the obvious beginning of the chain of events that lead to the Sumter Crisis, secession and ultimately the Civil War. (Hume, Video Lecture 4 & 5).
There were four political positions to be looked at in the forming of the Civil War. First, popular sovereignty let the people of that particular state or territory decide if they wanted slaves and for the government to stay out of it. Second, the Wilmont position, Free Soilers, held that slavery should be contained and should not be allowed to expand into the new territories. Third, the Southern Nationalists position held that slavery could not be excluded from the territories. The territories belong to the Nation and the 5th Amendment guarantees that you cannot lose property without due process of the law, Congress cannot arbitrarily deprive Southerners by drawing some line out in the territories somewhere and telling them that they cannot bring their property into the territory.