Question

Did Polk provoke the Texas-boundary conflict in order to gain California or expand slavery, as war opponents like Lincoln charged?

Answer 1

No, the Mexican American war happened because Mexican soldiers murdered American soldiers in cold blood on the disputed Texas-Mexico border, Mexican troops entered the US and killed American troops--which is an act of war. American troops entered the disputed territory, prompting shootings. Polk just sold it off as Mexicans killing Americans on American soil which in fact was not the case at the time. This is why Lincoln makes the "show me the spot" demand. Its still an act of war no matter how you slice it. Mexico was on the verge of civil war at the time, and there were more Americans living in the Mexican territory (especially Texas and California) than Mexican citizens. This territory was certain to become US territory. In addition, Mexican troops did indeed cross the Rio Grande (The internationally recognized border between Mexico and the US) and kill American Troops. That is an act of war and it is one of the principle causes of the Mexican American War. True, however that territory was still under Mexico's rule at the time, even if it was destined to become part of America later.
In addition, it was the US that first sent troops into the disputed area. From the Mexican stand point, the US invaded Mexico without its consent and this too is an act of war. The act would be on the US declaring war rather than the other way around.

Answer 2

I wouldn't exactly say so. California wasn't really on the agenda in the election of 1844, as the discovery of gold there which magnified its importance was still five years off, and indeed occured near the end of Polk's term. The border dispute with Mexico was originally between Mexico and the Republic of Texas -- Polk did not *start* it, the Texans did. Polk ran, however, on the platform of annexing Texas, which was promptly done, and then the US "inherited" it.

Did Polk by wanting to annex Texas want to expand slavery? Maybe, a little. Polk was a Southerner and a slaveowner. But he was from Tennessee, which was not a very fire-breathing slaveocracy state, and he himself seems to have been the old-fashioned status-quo Southern Democrat, who favored continuing the Missouri Compromise and dividing the eventual Western half of the Union equally into slave and free states, like the Eastern half. Texas would naturally fall in the Southern half, so he probably saw some modest advantage in getting the acquisition of a Southern slave state going, and in ensuring that the South *had* room to expand to the West.