Are any and all actions justified if they can be defended as breaking the enemy's morale or reducing your enemy's warfighting potential? Are there any lines that should not be crossed?
Also, if you're going to argue in favor of facing the blunt realities of war then maybe it isn't the time to hide behind gentle euphemisms like "break the enemy's morale".
Second question first: I wasn't using a euphemism, or at least not trying to. The strategic objective of the Allies area bombing in WWII* was to break the enemy's morale, so that enough civilians would put pressure on their government to stop the war, and force them to stop. Either through civil pressure or revolution if the public got mad enough. There was never the idea that we'd kill civilians for the sake of it, or destroy homes because we didn't like the architecture or something. The idea was to so demoralize Germany, and later Japan, that they sought to end the war. Didn't work in Germany, did in Japan. "Dehousing" is certainly a euphemism, but "Strategic bombing of the civilian population to break their morale" isn't, I think.
First question:
That's harder, I think. It kinda depends on the stakes of the war, which is hard to talk about as an American because we go to war for sport and profit. Congress likes to talk about "Existential Threats to Our Democracy" but we haven't really faced one since the fall of the USSR, and that one is debatable. The Taliban and Iraq could in no way, no how, actually have destroyed our country in the 2000's rhetoric aside.
So let's talk about Wars, with a capital W. WWII, the Khmer Rouge, Some of the African wars last century, Yugoslavia, the Cuban revolution, Vietnam (from the South's perspective), Third Punic war, if you want to be the bad guy, the Indian wars of the 19th century west. Those wars WERE existential threat to at least one side. Losing that war meant the end of you culture and identity, genocide, often enslavement, loss of homeland the whole shebang. Communists are especially bad. You lose a revolution with them and more often than not double digit percentages of your population are murdered.
So in the face of that, is there a line you don't cross? Somewhere you say "Nah, I won't. I'd rather die, see my wife and children raped and enslaved and my homeland destroyed."? No, you do whatever you can, and whatever you have to and you win, or you die, because there isn't another option.
That cold reality: that in a real war there isn't actually a line you won't cross, is the real impetus for the Conventions and Treaties of the 19th and 20th centuries. They try to provide an avenue for wars where the threat of losing the war isn't existential to the nations involved and so there can be red lines not to cross. Those efforts are marginally successful, at best, as WWII shows, and as we would have demonstrated had the Cold War gone Hot.
So take the current Israeli-Hamas conflict. Israel is willing to have a two state solution, but not one Palestine "From the River to the Sea". They perceive the cost of giving up that much land and resources as the end of Israel as a viable state and a prelude to a Jewish massacre. Hamas is unwilling to accept a two state solution and would like to take over the the governing of (at minimum) large portions of what is now Israel and the people there, and make a Arab-Muslim state. (Maybe Theocracy, maybe just state with strong Sharia Laws, depends on which Hamas leader you ask). Both sides see losing the overall conflict as an existential threat to their people and homeland. So they probably don't have a line they won't cross, and we should stop pretending they should. This is not that kind of conflict. The only thing both sides agree on is this overall conflict is a war of survival. They might, given enough pressure from outside forces, negotiate an end to this particular flare-up, but they will not end the conflict until one of them is not able to continue.
Personally, I think Hamas is the "bad" guy here. Their basis for this war is unjust, I feel, but clearly they disagree, and are going to go at it full steam. Israel should make decisions predicated on the knowledge that Hamas will continue this war until they fall and there is one united Palestine.
* The Allies also had bombing campaigns targeting military or industrial targets, but those aren't the ones we're discussing here.
