The US military uses walkie talkies, but they are different from the ones usable by civilians. For starters, they use vastly different frequency bands than FRS/GMRS, often in the 138–144 and 225–400 MHz bands. They also use special military radios such as the Motorola SRX 2200, which are generally not for sale to civilians. You can monitor military traffic with some receivers, but unless you’re in the military yourself, transmitting on them is a bad idea.