It wasn’t through war or diplomacy or statecraft, or because of politicians like Andrew Jackson, Sam Houston or James K. Polk, or generals like Winfield Scott, Zachary Taylor or Phil Kearny. The lands west of the Mississippi became American because they were settled by Americans. If you’re interested, the story is best told by the premier historian of the west, Bernard DeVoto.
On this date in 1843 a part of that saga began. Here’s the link to the story.