My earliest ancestors came over in 1620, the last came over in the 1850s. So during the civil war, all my ancestors were already on this continent. However, none of them participated in the war since they all lived out west, beyond the boundaries of the USA, throughout the war.
The West was involved in the war in various ways. California was a state and Nevada became one. Gold and volunteers were sent east. There were troops sent to secure New Mexico from the confederacy.
The civil war took place after the Mexican-American war and the establishment of the border with British Canada. What is the western US now was under US control. Some parts were states, some territories. There was military and political activity all over the region related to the civil war. My ancestors lived in Utah. They didn't go fight, but that doesn't mean they were entirely isolated from the events.
In gangs of New York they portray immigrants getting off ships, being recruited and then being put on other ships to head south. Doubtless there's a good dollop of artistic licence involved but it possibly was a thing
