Plausibility: Seperate Military and Critical Infrastructure Internetworks?

Delta Force

Banned
ARPANET was originally developed for military and government purposes before becoming part of the internet backbone in 1990.

I'm wondering if it might have been possible for ARPANET and/or other systems to have remained as an independent network with no connection to the wider internet. This would be in following with the air gap principle, in which systems holding sensitive information are kept physically separate from those with more sensitive information. In this case it would be taking the principle to its ultimate conclusion, creating an entire network just for military and other critical activities such as national defense contractors, energy and water utilities, etc.

While it would not eliminate the risk of hacking or sabotaging systems connected to ARPANET, the system would only be accessible at infrastructure and facilities that support its operations. Someone might be able to break the air gap by plugging an ARPANET computer into a system connected to the wider internet, but it would be traceable to a physical location within ARPANET and could thus be isolated from the network while government/contractor security search for the source of the breech at the impacted facility.

Would something like this be a feasible option for securing military, contractor, and critical infrastructure networks?
 
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