Among the officers of the Mexican-American War, Stephen W. Kearney was one of the leading personalities. More than anyone else, he was responsible for the conquest of California, which he subsequently administered as Military Governor.
The appropriation of so much territory from Mexico was entirely in keeping with the ideology of Manifest Destiny, which held that the United States had the right to expand its territory across the length and breadth of the North American continent. Manifest Destiny was a key driver behind the Mexican-American War. Mexico stood in the way of American expansion, and so had to be dealt with at some point. As the Mexicans wouldn't just give up their territory, the Americans were left with no option but to fight if they were to achieve their destiny. And fight they did, achieving victory after two years, thus adding vast swathes of territory to the United States.