Imperialist slaveocrat. HP.
Despite the fact that that none of the territory he helped annex ever became slave states and in fact California strengthened the power of the free states.
The intent (and Polk's position) was to continue the 36-30 line to the Pacific Ocean (and the AZ/NM territories were settled by slaveholders and tried to join the Confederacy). Had Mexico kept its territory, slavery would have to have ended at Texas. And of course, it was a blatant war of aggression (and Texas only seceded from Mexico because they wanted to own slaves).
The fact that there were rebellions throughout Mexico and not just in Texas suggests that slavery was not the only issue. The republics of Texas, Rio Grande, Yucatan, Zacatecas - the common thread is a centralizing government and the abandonment of the Constitution of 1824.