This is the only reasonable argument, but I know rabid liberals frothing at the mouth over more dead Russians don't want to see it.
Still, for those who are still reasonable in their goals and ends, and the outcome they actually want to see, there's only one solution.
The argument on the pro-UA side is that Putin has the "choice" to just up and leave Ukraine at any time. I've written before about why he doesn't -- not after all the sanctions, not after all the resources and troops committed. NATO/Europe will not say "alright you're a good sport mate, we'll drop the sanctions and pretend this whole thing never happened, deal?". There's a "warrant" out for Putin at the ICC, what makes anyone sincerely believe things can go back to where they were before for Russia?
It should hopefully be clear that the people calling for Putin to just "leave Ukraine" know they're not being reasonable. These are some of the same people who also cheer when Ukraine gets new weapons to kill more Russians with. The goal for this segment is not peace, it's actually to prolong the war.
I'm addressing the reasonable side, however, who might have been inadvertently taken in by this psyop. I mean, isn't it weird how NAFO suddenly popped up everywhere and got media coverage? After they're doing the exact same thing as the Lithuanian elves (because elves fight against orcs) did back in 2014, and it turned out the "elves" were NATO?
Saying Putin can just leave Ukraine is impotent rage. Yeah, sure, I'm just gonna give him a quick call. He really cares what some westerners think about his war, especially now that he has to contend against NATO intervention. I'm sure we can totally convince him he's being a total nerd if we just yell about it on Twitter.
So if you actually want peace, what's the reasonable solution?
Organise locally, petition your government to stop sending money and weapons to Ukraine. Organise and protest for a peace deal. I know it's probably not what you want to hear.
But these weapons prolong the war. Just earlier today I saw a video of an APC full of 8+ Ukrainians get obliterated by a missile strike. Where did they get that APC do you think?
Do you want Ukrainians to stop dying? Me too. The most pragmatic way, the only one towards which we can actually do something, is to protest against our governments sending equipment that prolong this war.
Otherwise you have to realise this war is going to be fought to the last Ukrainian. Do you really think it's weakening Russia in any lasting capacity? The fighting is happening in Ukraine. Ukraine has been losing population consistently since 1990 (a whole fifth of it lost, mostly, to emigration). What exactly is weakening in Russia? It's Ukraine that's being destroyed and will need to be rebuilt. It's Ukraine that's losing their population by the thousands. It's Ukraine that's shooting depleted uranium bullets on their own soil.
What you're advocating for, when you advocate to prolong this war, is to kill every last Ukrainian and turn their country into a wasteland. The UA army is even using depleted uranium bullets now (courtesy of US and UK), which have polluted the land when they were used in Iraq. This is backed up by data, right? Depleted uranium has data behind it, it's not speculation.
The reason the war is being prolonged is because it makes a lot of money to the military-industrial complex. It makes a lot of money to corporations due to the inflation. A blackrock affiliate said on hidden camera recently that when Russia blows up a grain silo, the price of wheat goes up. War is a great business opportunity, always has been. And then when Ukraine will need to be rebuilt, it'll make Blackrock a lot more money.
But I'm not sure who they will rebuild the country for if there's no one else left to live in Ukraine.
My solution is the only one that makes sense. Any other is purely impotent and just venting frustrations, not effecting any actual change.
The USA has global hegemony right now, while Russia doesn't have the economic capacity to be a hegemon. The USA has been bombing and destroying countries for its entire existence and is currently using Ukraine to try to do the same to Russia. It is powerful enough and dangerous enough that nearly any action that opposes US hegemony is a good thing for the rest of the planet. It's already caused enough damage that it is now fundamentally an existential threat to humanity.
In this lens, who does it make sense to support? Russia, who has repeatedly asked NATO to not expand eastwards after its application to join NATO twice got rejected, or Ukraine, who had a US-backed coup back in 2014 that was unpopular enough that Crimea and the Donbass voted to leave Ukraine? A coup that replaced the democratically elected pro-Russian government with an anti-Russian one who has been shelling the Donbass for almost a decade. The Ukraine border is geographically the hardest for Russia to defend (which is why they aren't particularly worried about Finland joining NATO), a weakness exploited by Operation Barbarossa. Which side has Azov again?
We support Russia because we support any and all actions that bring an end to the war in the Donbass as soon as possible. We support them because it was Ukraine who violated the Minsk agreements, proving that they can't be trusted to not push into Russia and extend its shelling of russian-speakers in the Donbass into the entirety of Russia as soon as they somehow won against Russia. More importantly, Russia losing would mean it at best becoming a puppet state to the US and at worse being balkanized. Both outcomes would be objectively bad for China and every country that currently depends on Russia to resist US aggression (Cuba, Iran, Syria, everyone currently being sanctioned etc.).
We don't support Russia because we think it's good or is a worker's paradise. We support it because supporting US-backed Ukraine is objectively the wrong position to take. You can't ignore a nation's national security concerns and not expect them to react violently when said concerns are violated.
Liberals have such a hard time thinking in anything other than a black and white moral lens when it comes to the adversaries they've been conditioned to hate, yet they reserve a bunch of grey space when it comes to their own history and figures, even as those same living figures continue to commit horrendous atrocities in the name of capitalism.
Like you said, recognizing that the US is the greatest terroristic hegemon on Earth at the moment means that Russia losing this war would objectively be bad for the future of the global south and socialism at large. If Ukraine becomes a NATO state and Russia becomes further balkanized, these "progressives" will never get their wish for the uncorrupted socialist paradises they fantasize about. Acknowledging that means subscribing to material reality, not because we "stan dictators" of whatever the fuck. It doesn't mean we like that two countries who essentially used to be siblings are forced into a meat grinder. We want it to end and we recognize who the real aggressor is.