Herfried Münkler makes some points in his book from 2002, "Die neuen Kriege", that challenge your assumption that expanding borders through war is normal. However he seems to be very Europe-centric, which my answer is, too.

First I want to challenge the assumption that nations with fixed, clear cut borders are normal. Nation states are rare, relatively speaking. Münkler contrasts nation states to empires (or maybe realms is a better translation of Reich in this context), which have no clearcut borders. Instead they fade out into disputed regions, often defended by some subjugated and/or allied tribe. The condition of war and peace is not binary but more like a gradient, being more peaceful in the center/capital and getting more violent towards these so-called borders. Examples are the Osman Empire or the USA, before the Pacific was reached and before the border to Mexico was established, or the Danubian Monarchy in the east. A nation state or modern territorial state on the other hand has fixed borders with a quite clear cut - because official - difference between war and peace.

The other point he makes in the same chapter is about the lack of nationalization of war before territorial states were established. He specifically mentions wars of the late medieval and early modern history. He states that usually, there was no possibility to capture fixed positions, keeps, or wall-protected cities. Instead the surrounding territory would be devastated, the villages and farms being burnt down. Pitched battles were rare, and he argues this point with the relation of attending warriors/soldiers to the fallen ones, which rapidly rose from 5.7% in the 15^th century to 15.7% in the 17^th century. The goal was not to conquer, occupy or politically annex new territory, mainly because that was not possible due to lack of personnel and funding. Instead the goal was to cause enough reliable damage to coerce the opponent into treaties, to fulfill certain demands.

So together these two points show that politically annexing territory and incorporating it into your nation state through war is not a normal way to expand your borders.

Edit:

I think my main problem is that I thought OP limited the question to (european*) territorial states due to him mentioning Russia as example. I also understood "expanding" as "politically annexing territory". And finally, the "expand your borders" can be taken more literally in as much that only adjacent territory is meant by OP.

So the question is not that simple - does the British Raj in India count as "expanding"?

*more from western Europe to the iranian plateau in the Middle East, but it's still a eurocentric view as far as I'm concerned.

Isn't that what empires do? It seems like you're delving too far into semantics. Was British imperialism seen as abnormal or morally reprehensible at the time?

'Normal' probably wasn't the best term as its definition is so nebulous.

It seems to given the spirit of his question that it was with the establishment of the UN charter that it became viewed as unethical.