Well for the sake of a hypothetical scenario with my right of being totally wrong I'm curious enough to take a stab at it.
I don't think anything stops them.
Imagine if you will that an enemy has chosen the path of infiltration into a nation rather than a frontal assault of physical war. One would use their currency against the weakest link (state) in union. Pick the largest unstable state and control the politicians with easy money. Hell you can even bribe them or set them up like the mafia would. Do this slowly over time by eroding the family foundation and the education system. Protect your own by using a judge you bought off. Change some laws to spy on the people. Strategically rug pull a group of weak states. Attempt to abolish the Constitution.
Just a theory.