US Constitution, Article IV, Section 4:
"The United States shall guarantee to every state in the Union, a Republican Form of Government, and shall protect each of them against Invasion;..."
I don't think Invasion needs to be a formal army. Too many things point to the invasion of illegal immigrants from Mexico as an invasion that can destroy state, if not Federal sovereignty.
Why is our Federal government not following this article of the constitution?
We have lessons from our history. In the early-mid 19th century, how did the United States get the Oregon Territory? Was it by war? No, there was threat of war over an explicit boundary line, but the victory was simply by sending so many American trappers, hunters, farmers, merchants, etc. into the territory that the British knew they had lost it.
Isn't that what is happening now?