When the U.S. subjugated the Indian nations, what was the impact of this western expansion on national unity?Explain your reasoning and support...

There are two major ideas/names to keep in mind when
thinking about this question.  They are manifest destiny and Frederick Jackson Turner. 
These two show how the subjugation of the Indians and the westward expansion that went
with it promoted national unity.

Manifest destiny is, of
course, the idea that the US was assigned by God to spread across the continent.  The US
deserved this because of its superior government, culture, and "blood."  Turner argued
that the frontier is what defined America.  The frontier gave Americans an identity as
tough, independent pioneers.

Between them, these things
increased national unity.  They created an identity that all Americans could share. 
America was a country singled out by God to fight against Indians and others on the
frontier as a way of improving the world.  This view of America's mission helped to
boost national unity.