Question:
"What happens when the federal government no longer protects the states against invasion?"
The Constitution on the matter:
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
When the DC/fed/communist government no longer is willing to fulfill its duties under the Constitution, or acts intentionally to subvert such duties, it has become time to re-interpret the 10th Amendment, and for the states and "the people" to take up the burden of those duties upon themselves.