Sitä voi voittaa useita taisteluita, mutta hävitä sodan.

English Translation

One can win several battles but lose the war.

What’s the function of “Sitä” at the beginning of the sentence?
The sentence uses nollapersoona, so “sitä” is not a subject, and the verbs already have objects.
As I see it, you could omit it and the sentence would still be ok and mean the same.

So it must be some kind of filler word that makes it more idiomatic and natural?

Which would of course not be a very satisfactory answer, but what can you do.