It’s my understanding that the federal government is expected to assist when a state needs help repelling an invasion.
However, the initial defense lies with the state itself. A state may not “engage in War unless actually invaded” but I think Arizona could make a pretty strong case that some of these fence-jumpers are FAR more dangerous than humble lettuce-pickers in search of employment.
“A well-regulated militia being necessary to the security of a free state” gives us a hint where Arizona could get enough troops to close the border and stop the invasion.
Article IV, Section 4:
The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.