Is it possible that an adversary to implement a backdoor into a chip design, without Apple noticing it?
I'm not a chip designer so perhaps the answer is obvious to some of you guys, but I'd expect some verification mechanism at Apple's side of the manufactured chips to match their original design to verify that they aren't tampered with?
Manufacturing chips in the US means the DoD can investigate the acutal fabs and put cleared personnel on the manufacturing line to make sure nothing untoward is going on. Another strategy is to investigate the chip after it's been manufactured somewhere else and prove that it's the same chip you designed, but that's quite difficult.
If you're interested you can read up on the Trusted & Assured Microelectronics (T&AM) program.
If an adversary wants to deny access to a fab on American soil they'll need to deny access to dependencies or attack the fab itself.
Apple has a top notch logistics and security processes which had mitigated the issue of supply chain attack in China which his willing and capable of producing such attacks.
Moving some production to the USA might induce some sloppiness in this due a perceived inferior risk.
Also, some security measures requested by Apple to manufacturers in other countries are probably illegal in the USA.
That's interesting...do you have any specifics?
I'll take a guess and agree with TOMDM. It's about China invading or blockading Taiwan (remember the US attack/blockade against Cuba? Exactly that.) and thus denying America physical chip shipments.