This is the 10654th most frequent Korean word.
군대로
"군대로" means "to the military" in English.
Here, '군대로' signifies the destination (military) someone is heading towards.
저는 다음 주에 군대로 입대합니다.
I will join the military next week.
In this case, '군대로' refers to the origin of the letter being from the military.
철수는 군대로부터 편지를 받았다.
Cheol-su received a letter from the military.
Within this sentence, '군대로' indicates the direction of movement (towards the military).