The military have been using long-range two-way radios for nearly 100 years already, and naturally, the technologies used have changed a great deal in that time.
The first military radios were morse code transmitters and receivers, used in the trenches of the Great War in Europe, which nowadays is referred to as World War 1 (1914-1918).
Armies began to use 2-way radios to pass messages between the front lines and their battalion headquarters, because the army field telephones needed to have wires laid between both stations. And these telephone wires were easily damaged or cut by enemy action.
But wireless could still get a message through.
The early army radios were crude transmitters and receivers which only sent and received morse code signals. They needed large and heavy batteries to power them, their antennas made them prime targets for enemy snipers and artillery. What is more, the enemy would try to listen in to their radio signals and make sense of the information. So signals had to be coded or enciphered, which took time to put into code and more time at the other end to decode each message.
The world’s navies made good use of their long range two-way radios to send coded messages between ships, and to naval stations on shore who could relay those messages back to the home base.
And in both World War 1 and World War 2, Germany used its long-range radios to stay in touch with their U-Boats (submarines) which attacked convoys of supply ships and enemy vessels (usually British or US) that they hunted and destroyed very successfully.
And while military radio transmissions are always at the mercy of jamming attempts, the signallers found they could change frequencies without too much difficulty. (Modern military radios can do automatic frequency-hopping, controlled by an inbuilt computer chip, which allows the radio sets in a network to stay in touch but which makes it extremely difficult for an outsider to follow as they keep changing channels back and forth all over the place.)
By the time of the second world war (1939-1945) two-way radios were much more sophisticated. Most radio sets could use voice (amplitude modulation, or AM) as well as morse code (continuous wave, or CW).
Single Sideband (SSB) mode of transmission was not invented until the 1960s, I believe.
Unlike World War 1, where air force pilots had no radios (and no parachutes), military aircraft could stay in contact with their bases over distances of hundreds of miles – and the pilots could talk to other planes while they were in combat.
The military radios had been made smaller so they were more portable. Voice transmissions were used mostly for short range contacts, while CW or morse code was able to get through clearly over much longer distances.
Even spies could operate from enemy-occupied countries with small suitcase-sized radios and tap their secret messages back to their home base. They were at great risk from counter-intelligence forces who used radio detector vans to pinpoint where a transmission was coming from, and would rush to find and arrest the secret agent. And spies who were caught were usually tortured for information before being shot.
Modern military long-range 2-way radios include various army packsets which cover most of the HF (shortwave) bands. To stay small enough to be carried as a backpack, they usually have a maximum power of 20-25 Watts and cover around 200,000 possible channels between 2-30 MHz. Most can handle AM, SSB and CW signals and some have digital and encrypted modes, plus ALE (automatic link establishment) and frequency-hopping technologies.
Some licensed radio amateurs actually collect these military radios and use them, all perfectly legally. Secondhand military radios can be bought legally in the USA and in the UK. The problem is they will often need some repairs, and you probably have to make your own power supply or battery packs to suit. Also, even the newest ex-army radio sets are quite heavy so they will be very expensive to send through the post or by courier. You will occasionally see one on sale in eBay. But it is a case of Ceavat Emptor, which means buyer beware!
If you do a search for Green Radios or Military Radios, you will find more information. There are also very active groups to be found in Yahoo Groups.
I haven’t even mentioned Satellite Communications, which are very secure because they use satellites which are owned and controlled by the military themselves. However, the satellites are so expensive that only the biggest and richest of governments can afford their own satellites. So many armies, navies and air forces still use radio for their long range comms.