The Northern Line Army Group is different from the Western and Southern lines.
The Western Front has an absolute main force: the Otaubis Battle Group, the Solar Fleet, and the Sun Legion.
Although there are three entities, they are all direct units working for Holy Terra, and they have been cooperating with each other for a long time. Salius has full leadership authority over the Solar Fleet and the Sun Legion. Whether it is the Fleet Commander or the head of the Star Realm Army, they have no resistance to following Salius's orders, allowing him to fully mobilize the Western Front Army's troops.
As for the Southern Front, it's even less so, as it's only the Alliance itself.
