A Windows XP machine's life expectancy in 2024 seems to be about 10 minutes before even just an idle net connection renders it a trojan-riddled zombie PC
The biggest vulnerability in most computer systems are people. Many Soldiers are too lazy to save or encrypt thier files and lose them or comprise them all the time.
A computer system that isn't designed to and doesn't need to connect to the World Wide Web should be safe. If no one, for example decides they want to access a certain civilian website and so finds a way to connect their terminal to the World Wide Web, compromising it.
I am more referring to the U.S. Army intranet system. Which while some services can be accessed from the internet, some can only be accessed while connected to the Army Intranet. The Amry uses alot of terminals, computers, that have no need to connect to the internet but only the Army intranet.
Also, the Army modfies some of the programs and hardwere they use, such as Windows from their standard commercial versions, removing or changing features and such.