1. 80% Of Immigrants Come From Asia Or Latin America
2. Immigrants Often Start Their Own Businesses
3. There are 41 million foreign-born individuals living in the United States
If people did not immigrate to America then there would be less people in the U.S then now. If there were less people in the U.S then that would also mean that there would be less crime there. Not much wars would happen in the U.S if people didn't immigrate there.
It'd allow the North to control goods going down the river to the South. Doing this, the North could cut off food or weapon supply to "starve out" the other side. The North could also use the Mississippi for transporting their own troops into battle.