Just read Jeff Shahara's "Gone for Soldiers" about the 1846-1848 war between Mexico and the US. Fiction but based on the events, diaries and accounts of the people in it - and a good read.
Anyway, the book asserts that the loss of Texas was a huge blow to Mexican pride. 1845 Texas becomes a state and Manifest Destiny is in full bloom. The Texans consider their border goes to the Rio Grande where as the Mexicans begrudgingly feel the border stops at Nueces River. Both sides start making moves into land they feel they control. Polk sends a force of three thousand under Zachary Taylor to protect US settlers. This only acts to further enrage the Mexicans resulting in a declaration of "defensive war" with the US. Of course, the battles begin in the disputed territory.
What happens next is the most interesting. Polk talks to a representative of the then exiled Santa Anna who promises to negotiate a peace. Santa Anna is allowed to proceed through an American blockade of Mexican ports where he immediately gets control of the Mexican military again.
Instead of peace, Santa Anna goes on the offensive against the US.
Ultimately, this would lead to Polk sending the expeditionary force under Windfield Scott and the ultimate capture of Mexico City.
I also learnt that Robert E Lee, Pierre Beauregard, Perry (later Commodore), Ulysses S. Grant, James Longstreet, and Thoms "Stonewall" Jackson all fought under Scott in the campaign - incredible.
So in summary, land grab- yes, initially at least, but i think the US did a lot to try and stop the war from progressing. Santa Anna's duplicity (and Polk's naivety) played a huge part in the final result.
Apologies for the long post...