I can see both happening.
If foreign troops land on American soil, a few things should probably happen:
- Definite increase in manpower
- Add militia divisions to numerous provinces (these would be your average Joes taking up arms)
- Dissent increase
- Industrial shift (kind of like the Russian Move Industries to Siberia)
- Fortification of coasts and cities
- If enemy troops are near, movement of the capital to somewhere else (not sure where, though)
- If a large number of national provinces have been lost, then there should be a leader change, or possibly even a revolution.
I'm going to code all these things and post in the main events thread, as well as revolution event, because I think America needs one. Let me know what you think of the other specifics.