It chose not to, because the cost exceeded the benefit in the specific circumstances of 1868.

That choice was a reversal of a policy that the United States had been pursuing, and it demonstrates that specific decisions — not historical destiny — shaped the expansion.

If the circumstances had been different — if the Lakota resistance had been less effective, or if the Montana goldfields had been more economically important, or if the political conditions in Washington had been different — the outcome might have been different.

This means that the outcome was not ordained.