

The Republican Party was predicated on continuous western expansion. It was the successor to the Free Soil Party in the west and what was left of the Whigs in the East.
That necessarily meant seizing more land from American Natives and distributing it to Settlers. Much of the Union Army, before and after the Civil War, was focused on decimating the Native population and securing new tracks of free land for settlers. Lincoln inherited that mandate when he took office and pursued it as zealously as any Republican before or since.
The Dakota War came out of a strategic starvation campaign imposed by the Union Army over Sioux Territory. The original tribes had been forced off the productive soil around the Minnesota River and displaced into barren wasteland. Subsequent crop failure and long winter made trading for foodstuffs from their home territories the only means of survival. And the settlers took maximum advantage, deliberately scamming and price gouging the Sioux for the remains of their family wealth. This, after a series of treaties had been casually violated from administration to administration.
The war was quite literally a fight for survival by the Sioux. Lincoln’s largess in hanging only the young men directly involved in the raid did nothing to prevent the Sioux population from continuing its rapid decline, as the surviving elders were left to starve to death in the wilderness and the children were forced into Christian schools notorious for brutalizing and killing the kidnapped youths.