While the extent of Texas was a patriotic issue prior to the Mexican-American War, after the war's conclusion it immediately became a question about slavery. The state's borders would be heavily debated until the issue was resolved in the Compromise of 1850, but this conflict heavily tested the cohesion of the American nation.
After the war, the question of whether the newly-won territories would be free or slave was raised. Zachary Taylor, despite being a southern slaveholder, believed that the new territories should be free, as they did not yet have a history of slave-ownership. Texas had been accepted as a free state, but its borders needed redefining. After much debate, Congress settled on the presently-known shape of Texas, with its northern boundary defined by the earlier 1820 Missouri Compromise, and its western border having been extensively debated. This reduced the size of Texas by as much as one third.
The political boundaries of this compromise may appear to heavily favor the abolitionists, but the southern states won some large concessions. Most prominent among these was the passage of the Fugitive Slave Act, a far-reaching act which abolished trial by jury for black citizens accused of being escaped slaves, as well as requiring all impressed northern citizens to participate in the capture of slaves. This bill was extremely pro-slavery and stirred abolitionist tendencies in the north. The success of this act in achieving its goal is easy to see in its economic impact: the price of slaves rose fifteen to thirty percent more in border states (where the issue of escaped slaves was most prominent) than in the deep south, reflecting the added security of investment given by this act.
This compromise would have far-reaching consequences, even if, at the time people, believed that it had saved the Union. Not only would it lead, almost directly, to the Civil War, but it also indirectly led to the strengthening Supremacy Clause, reduced territorial ambitions for Mormon settlers, and larger Indian reservations in the state of Oklahoma.