I'm confused. If AZ starts defending it's borders, it would be doing the job originally delegated to the US in ArtIV sec 4 as you posted. Unless they were contracted by the US to do so wouldn't AZ be reclaiming that power?
Or is the illegal immigration issue not severe enough to call it an invasion?
AZ has always had the right to defend it's border. There is no reclaimation of power. Art.IV, Sec.4 says the feds
SHALL defend AZ and all other states against invasion.
The power is not delegated, it is demanded, think "shall issue" as in concealed permits. The feds have not acted accordingly, so AZ can and should defend it's own border. Az, Tx, NM, Ca, and all other states that have an over abundance of illegal immigrants should file suit on the feds for failing to perform it's duty.
If 11,000 people a day (IIRC) is not an invasion, I want someone to tell me how many it takes.