They were invited by the local authorities.
You are new, so you probably do not even realize how unprecedented that is, but it is exactly that.
PIE have the support of local stations, orbital control, planetary resupply resources, and government communications. We have what we can do inside Yulai Accords and by very spotty high-orbit drops. They can land new armies. We can barely talk to ours.
If you are looking for a fair fight, you are in the wrong system.
Else
Quite tired by now