Likely rank dependent and a reflection of reality on the ground.

I don't know if it's a direct correlation to the above principle, but there is a bit of a myth out there that "I was obeying orders" is not a defense. To this day, the default position is that "I was obeying orders" -is- a defense against any offense and it is up to the prosecution to prove that that position should not apply. See, for example, the current US Manual for Courts Martial rule 916(d), especially when combined with Part IV (16) (Article 90) a (2) c 2 (a) (i) which states that any order is presumed to be lawful. I'm not familiar with the regulations of other Western nations, but would not be too surprised to discover they have analogues. Talking to a Swedish officer, they certainly do. It comes down really to 'how obvious was it that the order was unlawful?'

The head of the German military could have had the moral choice to start 'the war of aggression' or to resign. We expect our senior leaders to do that sort of thing. However, the further down the chain you go, the less discretion there is available for soldiers to refuse orders which may be a little less clear-cut at the time. Sergeant (Feldwebel) Snuffy cannot 'resign' instead of carrying out an order to invade another country. As an analogue see the case of then-1LT Ehren Watada, in 2006 who ended up getting court-martialed on the matter.

We don't expect low ranking personnel to be au fait with the fine details of international law and ethics over jus ad bellum (The 'right' to go to war). It is not clear-cut to Feldwebel Snuffy (Or whatever the German equivalent to Snuffy is) that the war in response to the Polish attack on the German radio station (as far as he knows) is a war of aggression and a crime against humanity. He cannot resign, enlistments are for a set period. He has an oath to obey orders which he cannot know for a fact are illegal. Does he really have a moral choice other than to march into Poland pursuant to the orders of his superiors?