It won't come from the government protecting us against the invasion. It would come from the (most likely) few Americans who are fed up. And guess who'd be the first to be attacked and arrested by the National Guard when they're called out? American citizens.
I would HOPE that our Guardsmen and women could distinguish between American citizens and those who are attempting to overthrow the country.
Ways to tell:
Skin color
Lauguage spoken
Color of flag they fly
Language of signs they carry
In all cases THIS TIME, it's obvious.
Sadly, any noble revolutionary act to restore this Republic to it's Constitutional foundation will be met with violence by all levels of government. Those in power will command the National Guard to act only in one way -- to assure the power of the State. Any popular uprising would be seen as a threat to the state and be met with incredible violence. To truly restore this Republic to its basic Constitutional role would require about 80% of the Federal budget to be eliminated, most cabinet departments and millions of workers fired. The only place I can see that happening is in some Mel Gibson film, not the real world.