Treason is the crime of aiding enemies of the United States during times of war, or in waging war against it. Only United States citizens can be guilty of treason against America. This is because treason is considered a betrayal of the nation, and one must belong to the nation in order to betray it.
Treason is considered an extremely serious crime in the United States. As such, people who are found guilty of treason can be sentenced to death. Imprisonment and fines can be required if the death penalty is not given. During the Civil War, the entire Confederate Army was guilty of committing treason because it waged war against the United States. However, President Andrew Johnson eventually granted Confederate soldiers amnesty after the war ended. If he didn't do this, then any Confederate soldier could have been tried for treason.