Key Event Illustrating Manifest Destiny in the 1840s

In the 1840s, as the ideology of Manifest Destiny gained traction in American society, one key event exemplified the belief in expansion across the continent. This event not only emphasized the sense of American exceptionalism but also highlighted both the nationalistic fervor and the implications for indigenous populations. Which of the following events best represents this manifestation of the United States' ambition to expand westward during this decade?