How to argue that the invasion of Ukraine is wrong in an absolute sense?

"If you say that this invasion is wrong in an absolute sense, then all invasions are wrong"

"okay, all invasions are wrong"

"Wait if all invasions are wrong, then even border disputes are wrong, and so conflicts over territory are wrong"

"okay, conflict over state 'territory' is immoral"

"Wait wait wait, if you say that ALL state conflict over territory, then the violent enforcement of borders is wrong"

"okay, border enforcement is wrong"

"Now we're really off track: if enforcing borders is wrong then state control of lands is wrong, and states need to control lands and territory to enforce property"

"okay, the ownership of land is wrong"

"So you're saying that no one should get a piece of paper saying they own property? That charging rent and violently enforcing ownership is wrong? Gosh that would mean the whole state system was based on immoral feudal concepts, that it's a preservation of an oligarchy that has sin at its very core!!"

"okay."

In this war it is very clear who is the aggressor. Attacking another country just to increase the potential of your own country is wrong.

There will be no benefit from this war. If Russia wins, it will be isolated for a very long time bringing down its economic and human potential. If the war wouldn't be started both countries would have further developed successfully.

Now there will be millions of refugees, hundreds of thousands dead, maybe more millions injured and maimed, destined to poverty and traumatized generation. There is nothing good that this war achieves.

It's likely going to severely damage Russia's reputation and power long-term. And bombing the heart of Russian civilisation is a strange way to show their pride in their history and love for their Ukrainian brothers.

Seek first to understand, then to be understood.

Understand the Russian history, self and world view, reasons for their views, why they are concerned with NATO expansion, why they conduct their security as they do.

Ukraine is as much a justified defensive strike for them as it is an unjustified attack by most of the world. The public rationale are not the real rational and has been in the works for more than 10 years.

It's simple. Killing people and uprooting lives just for the sake of politics from some random Presidential palace is abhorrent.

A few people's political agenda is not good enough for a full scale war.

War has never resolved anything and American war expeditions are the textbook example to learn from. The resentment from war continues for generations. Ukrainian people and Russian people used to view themselves as closely related but after this war, they won't.

Probably the strongest argument is that annexing Ukraine only moves the Russian border toward NATO rather than maintaining a buffer, and that's the best case scenario.

The second strongest is that Ukraine could never join NATO as long as it had a territorial dispute, so Russia had a cheap option to prevent that which was working just fine.

It's unprovoked killing of innocent people. That should be enough.

The west has set a bad example in many ways making

https://en.wikipedia.org/wiki/Whataboutism

an appealing defense for the Russians just as it was in Soviet days.

It seems most Russians do think the invasion is very wrong, ethically.

Putin goes on about what Russia wants and needs as if that's super-important and everyone should listen and really care and respect his words, while he totally ignores what Ukraine wants and needs, as if it doesn't exist except as a tool/resource for Russia. It's the way bullies, psychopaths and serial killers treat people.

Most people would like things in life, but realize other people are people too, and don't just treat them as resources to be exploited/abused/killed. People who treat others that are locked up, usually for a long time.