If you are a (relatively speaking) ethically aware person, it’s likely that your ethics are focused on the reduction of harm, most particularly to sentient creatures and most generally to the biosphere which supports their existence. A sentient creature is one which can be harmed in a specific way: it can suffer, both acutely (as in the sensations of a sparrow being torn apart by a sparrowhawk) and chronically (as in the sensations of that same sparrow slowly starving to death during the winter).
Both acute and chronic suffering are endemic throughout the world of sentient beings, and human life in technologically advanced societies is quite unusual in the extent to which it is able to keep such suffering at bay, to treat it as something out of the ordinary. When we think about other animals it might be fun to be, we normally choose those which are relatively exempt from being preyed on by jackals, infested with gnawing parasites, paralysed and pox-eaten by myxomatosis, or casually gulped down in their hundreds of thousands by passing whales. We tend to use similar criteria when thinking about which other human beings it might be fun to be: it might be nice to be a dolphin, or a wealthy layabout in Berlin; it would be pretty ghastly to be a vole, or someone poor, weak and undefended in the middle of a warzone.
Wars are not natural events, or at least are not wholly natural events: you can understand the daily slaughter of the wilderness without recourse to political terms; and wars can also give way to ceasefires, cessations of hostilities which would be incomprehensible to a hyena. But by disrupting and dismantling the defenses of society against starvation, pestilence and predation, war brings human beings closer to the state in which other sentient creatures live: setting aside the numberless cruelties of specifically human invention, the horrors of war are most generally the horrors of being a living thing that can suffer and perish, that must fear for its life, that can and will be hurt atrociously and for no reason other than the need for survival of some other living thing.
At what point does the ubiquity and intensity of suffering in the natural world render meaningless the individual effort to reduce the suffering of this or that suffering creature? Perhaps at no point: kindness remains a virtue, no matter how bad things are or how much worse they may get. But it does render one kind of meaning unavailable, and that is the redemptive meaning that the rhetoric of “animal liberation” gives to the task of extricating non-human animals from the grasp of human power, need and appetite. Life on earth without us would not be a paradise, in any sense that we could recognise according to our own preferences for comfort and security over terror and pain. The departure of humanity would, in fact, leave the world devoid of its only remotely ethically attractive feature: the propensity of human beings to try to make parts of it nicer, for each other and for such non-human animals as they elect to care about.