On Treason
(For
@SuperHoosierFan and
@toastedbread)
There are only two forms of treason in the United States: 1) levying war against them, or 2) adhering to their enemies, giving them aid and comfort. It is unconstitutional for Congress to expand the definition of treason beyond these two cases. Treason is the only crime so defined and limited by the Constitution. There is a reason for that: it was the habit of the British government to broaden the definition of treason to allow for the execution of dissidents that were not actually dangerous to the state. The Founders considered this tyrannical.
Levying War
The first type of treason only refers to American citizens themselves actually levying war against the United States. For this standard to be met, there must be an "assemblage" of people. I.e., some random jackass committing terrorism isn't committing treason. Even a couple of random jackasses aren't committing treason. McVeigh didn't commit treason. He still got the short end of a needle - rightfully so - but he didn't commit treason. The Whiskey Rebellion was treason. The Civil War was treason. You might make an argument that those rednecks who took over the nature preserve in Oregon committed treason, but I'd find it hard to believe a U.S. Attorney would actually try to go that far, and even if they did, I'm not sure it would fly. Point is, the first type of treason requires a group of Americans to actually rise up in some sort of armed insurrection.
Adhering to Enemies
This is the broader of the two. As famed U.S. legal writer Charles Warren noted, everything that would constitute levying war, if done on behalf of an enemy, would constitute giving them aid and comfort, but not all forms of aid and comfort would constitute levying war. So this type of treason is actually easier to prove. However, the enemy must be a government that is in a state of open hostility with the United States. It's not enough to simply be an adversary. There must be a state of war or a rough equivalent. Americans who sided with and/or helped Nazis and Japan were charged with treason. The Rosenbergs were not. They were charged with espionage, instead. A treason charge against them probably would not have stood.
Treason is a strictly defined crime in American law, thanks to the Constitution. There are historical reasons for this. Because of this, there are a number of activities that would rightfully be considered traitorous, which would nonetheless not be considered treason.