Jane Fonda obstructed US weaponry in Vietnam preventing soldiers from firing on the enemy... but you're right. I think she got away with it with the distinction between preventing harm from the enemy and actively aiding the enemy or something.

I went on Google and about 30 people have been tried for treason. It said that no one had ever been executed by the federal courts for Treason, though, and I know the Rosenbergs were executed. Were they tried at a state court? My US history got all fuzzy when I graduated.