This notion of 'defending the homeland' is in many ways a modern concept.
Plebes did not view the local principality as 'theirs' - another prince coming to take over just means a 'choice between lords'.
Obviously, it's different if the Romans are coming into German tribal areas, but, 'state nationalism' as we undertstand it is a modern concept.
Invading people to 'spread an idea' was mostly an Islamic/Arabic thing, and then Napoloenic in modern era. Very rare in history.
And propaganda is usually pointless, it's obvious who is invading who - there is no 'media' to control the narrative within, obviously, the local folks in towns were instructed what to say but this is low-grade propaganda.