It would be silly to claim that Jackson was unaware or indifferent to the fact that the Mexican War had a great deal to do with slavery and with increasing the strength of the Slave Power. It was part of the contradictory nature of Jackson as a political leader and of the broad movement of Jacksonian Democracy. Sean Wilentz writes in Andrew Jackson (2005):
The most powerful contradictions generated by Jackson's presidency and legacy had to do with slavery, democracy, and American expansionism.
Jackson left office as President in 1837. So he had long been out of office when the Mexican War began in 1846, and Jackson himself passed away in 1845. That conflict is also called la Guerra Mexicano-estadounidense, la Guerra de Estados Unidos a México and the Mexican-American War. Yes, trolls and nativist zealots, Spanish-speakers don't use the English name for the war exclusively!
Speaking of the developments around the Texas Revolution of 1835-6, Wilentz writes:
The difficulties that surrounded these developments, especially in Texas, also reinforced the stresses that were afflicting the country and the Democracy by the mid-1830s. The results deepened the tragic dimensions of Jackson's presidency, especially in his second term. To Jackson, westward expansion was chiefly a nationalist and democratic enterprise: filling in Thomas Jefferson's empire of freedom, pushing back any possibility of Old World meddling in America's affairs, widening the opportunities for ordinary, virtuous, hardworking Americans to prosper much as he himself, a westward migrant of humble origins, had prospered. Jackson's vision of invigorated expansion was closely tied to his attacks on monied privilege over banking and the currency: opening lands, he wrote, to "actual settlers" and checking western speculation would curb the rise of a class of nonresident landlords and land jobbers, among "the greatest obstacles to the advancement of a new country and the prosperity of an old one." His was a vision of western settlement as a patriotic and egalitarian fulfillment, free of strife, bloodshed, and the artificial hierarchies that Jackson believed had no place in a democratic republic. (my emphasis)
Before his death, Jackson did give his opinions and advice on the tensions that led up to la Guerra Mexicano-estadounidense. In the debate over whether to annex Texas, which had applied for statehood, Jackson expressed particular concern about the hostile designs of Britain against the US, the schemes of his arch-enemy the secessionist John Calhoun and his old enemy John Quincy Adams. Robert Remini writes in Andrew Jackson and the Course of American Democracy 1833-1845 (1984):
But all these [various partisan political] questions, important as they were, paled by comparison to the Texas problem. Jackson could not rest easy until he knew that it had been satisfactorily resolved. Once Congress reconvened in December, the General charged after its members with demands that they pass a joint resolution for immediate annexation and thereby execute the will of the people as mandated by Polk's election. At the same time he kept his nephew, Andrew J. Donelson, informed of all developments, instructing him on what he thought should be brought to the attention of the Texas officials. "This you will have to bring to their view," he wrote in one letter concerning the "secret designs" of Britain to reduce Texas to a colony. "Remember, the word reannex," he added, "this hold forth," namely the right of the United States to Texas under the Louisiana Purchase of 1803. As for the Florida Treaty of 1819 which renounced Texas, that was a "nullity, not having the approbation of France and the citizens of Louisia," no matter what "that old scamp, J. Q. Adams" says about it.
Early in the congressional session a joint resolution was introduced for immediate annexation. This resolution required only a simple majority in both houses. The friends of Calhoun and the Tyler administration insisted on a resolution that vindicated the rejected treaty with all its connotations about slavery. They also demanded a provision requiring that the United States absorb the Texan debt. Again, many voiced doubts as to whether Calhoun and his allies really wanted the resolution to pass, arguing that they actually preferred the dissolution of the Union. The House took up the resolutionfirst, and, after a lively debate, passed it on January 25, 1845. Obviously many congressmen believed with Jackson that the people had expressed their view on the subject and wanted Texas admitted to the Union posthaste. (my emphasis)
Given Jackson's position in the nullification crisis, his successful battle against the Bank of United States and its wealthy backers and his general commitment to expanding democracy, I'm willing to believe that democratic and national-security concerns were the primary elements of the issue for Jackson. Although he did tend to personalize political disputes, so the fact that he saw dark designs of Calhoun and Adams in the matter also affected his position.
Still, it's important to remember that, although Jackson sucessfully defended democracy and the Union against the South Carolina secessionists, Jackson's notion of democracy was for white men only. He did not see the expansion of slavery as such as being an undesirable thing. To the extent that Jackson and the Jacksonians understood democracy as involving freedom for whites and slavery for blacks, it becomes hard to clearly distinguish to what extent support for western expansion in their eyes was also - or even primarily - support for the expansion of slavery.
An Index to Confederate "Heritage" Month 2006 postings is available.