MILNET was the name given to a network that split off from ARPANET in 1983 to create an internetwork designated for use by the U.S. Department of Defense. MILNET later became part of the DoD Defense Network (DDN).
MILNET is also known as military net.
Techopedia explains MILNET
MILNET was used for unclassified military communication. It remained connected to ARPANET at a small number of gateways so that it could exchange email, but could be easily disconnected in case of security concerns.