If the US government does not stop the invasion, the border states have constitutional authority to do so.
Constitution of the United States of America:
Article IV The States.
Section 4. The United States... and shall protect each of them against Invasion;...
Article I The Legislature.
Section 10.3 No State shall, without the Consent of Congress,... engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.