Correct me if I'm wrong, but my understanding was that the federal government is supposed to protect us from invasion, not the other way around.
This sign is an attempt to restrict U.S. citizens from open land within the territory of the United States, because the federal government is not fulfilling its responsibility to protect the borders. There is no question that the federal government is capable of protecting the borders. We still have a large military with sophisticated weaponry. The federal government, for reasons difficult to fathom, has chosen to effectively cede a strip of land up to 80 miles wide south of I-8 to Mexico.
This is why Arizonans are angry. It's their land (even where it is owned by the U.S. government).
Arizonans should be starting to give some serious thought to secession.