As others said, you usually need to move data. So, the typical solution for defense sector was to use high-assurance guards or data diodes. The former are like firewalls with an implementation designed to actually stop state-sponsored hackers rather than fill bank accounts keeping out the least skilled hackers. They do the latter, too, at the prices they charge. The data diodes are devices that allow data to only flow one way. There's at least two, use cases for that depending on which way the data flows.

https://en.wikipedia.org/wiki/Guard_(information_security)

https://en.wikipedia.org/wiki/Unidirectional_network