On This Day in History

[Pergamos]

I usually hear "crossing the rubicon" as doing a big thing, that can't be taken back.  I've never heard it as specifically heroic, although sometimes it is, in context.  I also don't hear it in specific context as ending a republic, or anything else about the results of the action, that's specific.  it's always just a big, irrevocable, act that commits someone to a conflict in a way they can't take back.

Invading Greenland would be Trump crossing the rubicon, destroying NATO and whatnot.