With the war in Ukraine, I think a field has been opened up for studying the philosophy of war deeper. First of all, I would like to state that I oppose the war in Ukraine and Russian government's aggressive measurements at full of my heart and although it seems like my question is related to the current wars, it is not. I want to get to the very bottom of the idea of opposing any wars. Many supporters of anti-war movements actually support the idea, without any questions, that all people lives are equal. What I mean by is that, for example, many of them would argue that war doesn't benefit who get into the war but benefit only a small minority. There is something that this idea offers us. Every person is equal, and I don't have to die for other people. Although it seems like obvious statement, I do not know how someone can actually can confirm this idea with applying philosophy. It just looks like a premise that people like to believe. On the other side, one can believe that "all people are not equal in value," so wars in theory, as the more valuable ones can use the more worthless ones in any kinds of endeavor, is not wrong. As I said, I am talking about wars in theory, I do not talk about wars in reality where the oppressors' ideology is much more different. How can people may actually philosophically argue that all people lives are equal?, and if it just a premise that people would like to believe, why others should not choose believe its counterpart, which theoretically may justify war and killing people?
Most of the world views war through the prism of Just war theory - at least to the extent that the political discourse and the existing body of the international world are grounded in this view. Pacifism, despite being based on a sound belief that all human lives are equally valuable, has somewhat of a bad reputation due its impractical nature, which has historically resulted in dire consequences - notably, the pacifist non-opposition to Nazism.
Just war is intended to protect people's lives
On the one hand, Just war can be viewed as a practical way of protecting valuable human lives, which would be destroyed, if the aggressor is not opposed (jus ad bellum - right to go to war). It also codifies the necessity of protecting human lives during the war - e.g., one cannot target civilians or retaliate against them, one cannot mistreat the prisoners of war or combatants that are rendered harmless (e.g., paratroopers in air, shipwrecked sailors, etc. - jus in bello, right conduct in war.)
Enemy combatants ("bad guys") have less rights
On the other hand, prosecuting a just war does mean that some lives are considered less valuable than others - notably those of the combatants for the aggressor's side, who are considered legitimate targets. This is firmly anchored in public discourse - e.g., the foreign volunteers who part to Ukraine in order to kill Russians are praised as heros; on the other hand, Westerners who a few years ago went to Syria to fight on the side of Daesh/Isis were treated as war criminals/terrorists, and their countries of citizenship were often reluctant to assist them in repatriation or saving them from execution.
State/Social/Common interest above individual ones
What might seem more problematic is that one's own combatants often go to fight against their own free will. This is usually justified by the public/national interest - an acceptable explanation in a communist or fascist state, but a one that contradicts to the principles of liberal democracy. In public discourse the moral dilemma is usually avoided via glorifying the combatants, praising their sacrifice, and giving as an example those who enlist to fight on voluntary basis. As an example, it is not uncommon to hear these days that Ukrainians are willing to fight and die for their country - whether true or not, this claim certainly relieves the conscience of the westerners calling Ukraine to fight to the bitter end, rather than cutting a deal with Russia.
Finally, the non-combatants "accidentally" killed while targeting combatants are discarded as collateral damage. The phrase tries to diminish what in reality might be rather high death toll - equal or exceeding the number of combatants killed.
Other approaches to war
Besides Just war and Pacifism, one could also cite Realist approach: it agrees with Pacifism that war is inherently immoral, but makes a different conclusion - that while in war one is not restricted by moral principles. It is probably a fair guess the powerful of this world, regardless of which side they are on, are realists, reserving the just war rhetoric for general public... but this is a guess, based on certain assumptions about the human nature - trying to prove it would inevitably border on conspiracy theories.
Yossarianism (after the name of the protagonist of Catch-22) - putting self-preservation, protecting one's family and friends above the interests of country/nation is another approach, closely related in pacifism, but more grounded in individual liberty. Indeed, pacifism is arguably shooting itself in leg by pretending to care about every individual... but without real bothering that the individual's own opinion may differ.
First of all the primary position of pacifism is a general rejection of violence based on the believes that every human life is valuable and that violence causes more problems than it solves.
And based on empirical evidence that's not all wrong. Look at any war in human history and they all come with tremendous suffering on all sides and negative effects from that can last for generations. That includes environmental damage like how parts of the battlefields of WWI are still uninhabitable, how it's completely normal for Germans to find unexploded WWII bombs with almost every larger construction site in a bigger city, how Egypt is apparently still home to 23 million landmines due to wars in the 70s. That includes social rifts between countries, within countries and may cause larger or smaller conflicts that go on for decades after the official war has ended. That includes trauma both for combatants and civilians, like massive increases in rape, hunger, crime, dehumanizing propaganda and practice, fear for your life and living on survival instincts disregarding all the moral guidelines you once cherished can all have devastating consequences for victims and perpetrators.
Violence is obviously not a good argument and either breaks people, mentally and physically and/or triggers violent reactions as anything to avert violence seems permissible when you're existentially threatened. Thereby perpetuating violence and causing even more harm. And once you created a precedent for "might makes right" it's hard to go back to a non-violent interaction, because a lot of trust is broken and rebuilding it takes time.
Of course some people might think that you can just "win" a war. But how does that look like, again violence is not a convincing argument. Like the U.S. had won several wars after WWII just to again "lose" them because the war never ended it just took on a more asymmetric approach which often was more effective and even more insidious. On the other hand if you just want to "wipe out" an enemy in it's entirety than you a) have to make your own soldiers drop all morality that they would be accustomed to in your normal society, like don't kill unarmed people, women, children etc. and b) the other side as well as their allies and anybody who would not want to drop these concepts will oppose you with literally everything they've got. Making it again nasty for everyone involved.
On the other hand non-violence is theoretically possible and even beneficial. Like not having wars means not wasting resources needlessly, starving people, traumatizing generations, killing people. There are a lot of upsides not to have war and it's not unimaginable for that to happen.
So yeah most people and states adopted a view of at least conditional pacifism where they generally argue that violence is not good and that it needs to be reduced or eliminated.
Now practically speaking while there are a lot of good arguments for it to be eliminated entirely there are unfortunately always those who see violence not as an evil but as a tool. Broadly speaking violence is a form of coercion and any form of government that isn't completely mutual (which would be a question of definition as to whether that would still be a government) is using coercive means. They often tend to use less, or at least less overt means of coercion and ideally the coercion is regulated and complies with generally agreed upon rules, but there are always people who don't want or even can't agree with the rules and so the application of them ends up being coercion and violence.
Which leads to the problem of absolute pacifism and that is that a universal rejection of violence would also reject the resistance of the oppressed giving the perpetrator free reign to do whatever they like apart from a moral condemnation that comes with little to no repercussions.
Now ideally the moral condemnation and the moral argument would be enough to convince them and the lack of arms would indicate the lack of a necessity for an armed conflict and the option to resolve a conflict peacefully.
The problem is unfortunately that it requires that the other side WANTS to resolve the conflict peacefully. Like diplomacy, negotiations, meetings and whatnot are very effective if both sides agree to meet each other on eye-level and resolve conflicts peacefully. But if one side doesn't treat the other as equal and instead wants to overpower them, then this approach comes to it's limits. So it's less that pacifism treats equality as a given (it might also do that, but that's a different point), but it's a requirement for pacifism. The elimination of violence ultimately needs an elimination of conflict and/or non-violent means to resolve conflict and ultimately you won't be able to manage that without treating each other as equal. Sure you can overpower your enemy but that only means that they continue to view you as their enemy and your order as unjust and oppressive and they would be right in doing so. So that isn't peace it's a frozen conflict.
Now the crucial problem is, if you deliberately reject the pacifist proposal of a mutually peaceful coexistence or even a cooperation. Then we enter a pretty dark gray area of adhering to pacifism even if the proposal is irredeemably rejected (as absolute pacifism would kind of have infinite patience in that regard) or to (ab)use the first good reason to go to war even so a peaceful resolution of conflict was absolutely possible, but you just WANTED to go to war and were looking for an excuse rather than a reason.
The just war theory, on the other hand, gets rid of the necessity for absolute pacifism but instead argues that there are just causes for going to war and rules that apply when being at war. This get rid of absolute pacifism's defenselessness (though there are already pacifists who would make that exception for themselves) and argues that you are indeed allowed to use violence to fend off violence towards yourself or others or to liberate oneself or others from an injustice. While still somewhat adhering to some of pacifism's ideals often labeling it a "last resort" and while trying to make war less nasty to ease the aftermath.
However in practice a just war is also a war. You also opened pandora's box in terms of justifying violence. On the contrary as it's a way to "justify" violence it's also abused by those who want to fight unjust wars. Like as soon as the idea of a war is floated the government no matter the system is going to lie about the causes, goals, motivations, resistance, morality and whatnot. Like even the Nazis create a false flag attack on themselves to "justify" their invasion of Poland. And the just conduct of war is also something that can rarely be adhered to as there's often no one to control it and the winner of the war could just retroactively declare their conduct to be just. Or use "yeah it's awful but less awful than a prolonged war" as a get out of jail free card. Which isn't even true, first war crimes might prolong the war because you provide propaganda ammunition for the necessity to fight the war and to keep fighting it even beyond a potential official end and on the individual level the victims of war crimes still suffered tremendously maybe even more than a prolonged war would have let them suffer.
So there is a good reason to argue that there is no just war, that individuals within a war are generally incapable to tell whether it's just beyond the immediate moral implication of the actions, which in any and all wars are unjust.
Like if you declare combatants and make them legitimate targets you've already crossed a line that is illegitimate in pretty much all peaceful contexts. And the further you remove the killer from the target the easier it is to simply lie about the status of the target. Like civilian or combatant might just be a misattributed label while in real life it's murder, regardless of the label just more heinous if the person on the receiving end wasn't even aware of the danger. So is that distinction really meaningful to the individual pulling the trigger and could they tell from their action whether they fight a just war? And if they can't how useful is that concept to begin with.
So at best the just war theory is applicable to localized small scale conflicts where you have an international community that supervises the conflict and holds the perpetrators responsible. As soon as it becomes a large scale conflict none of that is enforceable and the sides of the conflict knowing that may give rise to unimaginable horrors.
Though difficult in practice the pacifist is still right, without people volunteering to be soldier or letting themselves be coerced to be one there would be no wars. Like their superiors can shoot them, but that still only means that they are 1 soldier down, it doesn't work large scale, but that is somewhere between extreme courage and insanity.