Which they did try to reduce civilian casualties with the warning + waiting 5 days for any civilians to leave.
So they've shown intent to avoid non-combatant involvement already.
Dude, it doesn't matter if they offered free kittens to those who left. They are okay with slaughtering anyone who's left, combatant or not, which is enough to classify it as intentionally killing civillians, if carried out. This would not raise any eyebrows in medieval times, because there were no rules other than what your liege imposed, but in the modern world it's a clear-cut war crime.
To be clear, it's not like the act of invading itself is a war crime (though it might break other kinds of contemporary international law). The issue is treating both soldiers and civillians as targets. Is it risky having to distinguish? Yes, potentially, as is being on a battlefied in general. It is required though and there's no talking yourself out of it.
Not sure what more do you expect them to do at that point, or how after 5 days there'd still be 'massive civilian casualties' to somehow blame on Allen side.
I repeat once again, this is not in reference to the situation presented in the manga. There is pretty much no hope of them not commiting war crimes as they are in contemporary understanding for us - that's because they are a medieval level civilization with no such notions. Historically, when a settlement was taken, the invading army always raped, pillaged and killed to their hearts' content and that's about to happen here as well (unless the author decides to yet again break suspension of disbelief). In order to avoid this, their whole kingdom would require a massive paradigm shift and that takes literal ages. So I don;t expect anything from them. In fact, the author should not have even brought it up, other than perhaps as Allen's inner thoughts, since he's the only one you'd expect to even consider any kind of humanitarian approach.
