In the world of martial artists, it was very hard to imagine the existence of an army.
Martial artists were used to being alone and fighting alone, only occasionally fighting in groups, but those were also very small, with only a few people joining hands. If there were any more of them, they instead wouldn't be able to unleash the battle prowess that they should have, because they wouldn't know how to cooperate, and also wouldn't have someone to command them.
When Ling Han and the others received a transfer order, they felt it somewhat strange. These students actually became soldiers, and were going to move out with a great army to go on a punitive expedition against the Purple Moon Empire.