Lanling annihilated six Demon Banner families, not six Demon Banner territories, and he didn't send troops to occupy any territory.
Why?
Because there was no need at all!
Currently, he doesn't have enough manpower; he is already struggling to manage even one Flame Demon Banner. Moreover, more than a million square kilometers is already sufficient for him for now.
Most importantly, destroying a Demon Banner's Master Clan is easy, but thoroughly conquering a Demon Banner territory is so difficult.
The common folk of the Demon Race are full of fervor; killing them all is easy, but getting them to pledge loyalty to you is far too difficult.
And this time, Lanling has newly annihilated five Demon Banner families, capturing three thousand warriors, over ten thousand flying riders, more than fifty thousand warhorses, over thirty thousand Demon Leopards, and so on.
All these war gains need to be assimilated.
