Historically, the primary tactic of organised armies was to attack the enemy's infrastructure, including their civilian base. For instance, in ancient times, military campaigns typically ended with the winning side pillaging and razing the defeated enemy's city and taking its civilian population as slaves (the ones who were docile enough- the rest were put to the knife). That was the case for the more merciful campaigners. Others simply slaughtered everyone and burned everything (e.g. the Mongols).

In recent years, we have this notion of battlefield morality, where armies are supposed to fight each other and an invading force is expected to occupy the enemy's territory _without_ proactively harming its civilians. Whether this is actually achieved in practice or not is a different matter (it is not- civilian populations are still the ones that bear the brunt of any invasion).

The thing is, modern morality leans very strongly away from attacking civilians with militaries. Again, whether this is at all possible in practice is a completely different issue, but what is certainly not equivocal, or "arguable" is that civilians must not be attacked deliberately.

You will notice for instance that even when the US bombs weddings or other civilian targets (it's usually the US) they never, ever, under any circumstances try to justify the killings. Instead, they explain the civilian deaths as a mistake and try to explain the mistake (the wedding participants were firing their AK-47s into the air as per custom, the bride's gown looked like an Islamic oriflame, the groom had a suspiciously long beard, or something equally absurd).

In other words, you might consider that bombing a wedding to kill an enemy lieutenant visiting it is "arguable"- but you are very certainly into a minority.

And like I say, that's some very dangerous arithmetic. We don't accept that innocent people should suffer in order to harm a legitimate target. You seem to be arguing that it's just a matter of how many innocents and how many legitimate targets we're talking about. There can clearly be no definitive answer, which opens the gates for a lot of what we currently recognise as war crimes.

Edit: removed a bit about genocide, to avoid further misunderstandings. Apologies.