Was the Mexican War an Excercise in American Imperialism?

In Manifest Destiny and the Mexican War, Ruiz shows that the Mexican War was an exercise in American imperialism. According to him, the U.S. was picking on the weaker Mexico so that it could gain more territory. Manifest Destiny was what Americans used to justify taking land from Mexico. Manifest Destiny included thinking white Protestant Americans were better than every other race in the world and that Americans needed to spread their freedom and culture across North America. In The Mexican War: A Study in Causation, Graebner concludes that President Polk tried to purchase territory in the Southwest from Mexico, but Mexico refused. Polk then aggressively pursued his goals which he felt were in the best interests of the United States. He feels that since Mexico was weaker, and Americans were persistent in their drive to fulfill Manifest Destiny, war was the result.
I agree with both articles. I agree with Ruiz in that Americans had no right to just steal land from another country because they felt it was their destiny or that they were the superior race. However, I also agree with Graebner because Americans were rapidly expanding westward and had the energy and persistence to settle in the territories and make good use of them. Most of the territories were sparsely populated by the Mexicans. Since Mexico refused, America had to choose between just giving up or being more aggressive. They chose the latter and were met with resistance which led to the war.