Re: Presumbly the UK has similar plans
The problem is that when another country invades a sovereign nation, or indeed 'gives support' to a tiny minority that want to break away, the far-right nationalists are generally the first to respond. If the US was attacked tomorrow, I guarantee that the ultra-nationalist organisations would be the first on the front line. The clues in their name.
However, let's not pretend that the breakaway regions of Donetsk and Luhansk are all sweetness and light. The UN Human Rights Monitoring Mission in Ukraine reported in 2014 that there was growing lawlessness in the region, documenting cases of targeted killings, torture, and abduction. With beatings and attacks on supporters of Ukrainian unity.
Also in 2014 Human Rights Watch have said that "Anti-Kiev forces in eastern Ukraine are abducting, attacking, and harassing people they suspect of supporting the Ukrainian government or consider undesirable...anti-Kiev insurgents are using beatings and kidnappings to send the message that anyone who doesn't support them had better shut up or leave". Most of the ethnic Ukrainians did leave and they relocated further west.
And yes, Amnesty International have also raised concerns in 2014 about the volunteer battalions in eastern Ukraine. Saying that they act like "renegade gangs" and also use abductions and torture. But as I say, this is the problem when there are threats to your sovereignty. Those ultra-nationalist groups become useful muscle, because the desire to protect their nation is part of their belief system. I would say though, be careful with what you believe of that 'footage that came out of Ukraine'. Some has been proved to be either filmed within Russia or be footage of previous Russian encounters. Bellingcat described some of it as 'Dumb and Lazy' with corpses showing signs of having had previous autopsies (i.e. they are cadavers placed within a scene to fake photos etc.). It happens from both sides, but it's Russia's go to ploy. Having watched a lot of Russian TV within Russia, it's like an alternate reality. They present objectively and provably false things with a straight face.
As you say though, "As for actual war crimes, wait until the dust has settled".
I would note that the make up of battalions like Azov now are far more diluted of nationalist sentiment than they used to be. Russia invading the country has understandably stirred up the desire to protect the homeland in people of every persuasion, but even pre-invasion those battalions were made up of people from all walks of life and all beliefs. Far-right experts Vyacheslav Likhachev and Andreas Umland have both said that since 2017 the Azov Battalion has been 'de-ideologised' and openly right-wing radicals had been 'cleansed' from the ranks. It even has several Jewish members now and is considered a regular fighting unit by most observers (granted one with a fierce reputation).
Putin has been using the nationalists' playbook within Russia for years and since 2014 Zelensky has been playing that game too. I hate these nationalist games, but the problem you have is that threats to a nation will always increase nationalism, not decrease it. The Russian 'de-nazification' line is a joke. Putin is just creating more extreme views, in both Ukraine and Russia. But that's all part of the plan. Create a tonne of extremists as a product of your actions and then point and say "look, we said they were all extremists". It's a tried and tested playbook.
You could argue all day about 'who started' the atrocities in the east, but the fact is, pre-2014 things were mostly peaceful and it was Russian interference in both the east and in Ukrainian politics in general that caused most of the problems. Yes there was political unrest during 2013-2014 and there were protests both pro-Russian and pro-Ukrainian, but it wasn't all out fighting. Ukrainians and Russians consider themselves part of a wider Slavic family and the close bonds between the people kept things peaceful (or it used to anyway).
As a side note, you seem to be using the term 'ethnic Ukrainian' like it's some sort of big bad conspiracy by the Ukrainian population, which is weird to me. It's merely a way of describing a subpopulation within a country or region. The wider population of 'Ukrainians' is made up of ethnic Ukrainians, ethnic Russians, ethnic Belarusians, ethnic Moldovans etc. It's just a way of saying 'this Ukrainian has Russian roots, or this Ukrainian has Ukrainian roots'. They're all still Ukrainian citizens and we need some way of describing the subgroups.
Of course Ukraine has nationalists, but it has no more of a problem than other countries have, including Russia and the US. You can argue that France and Hungary have a far bigger issue with extreme nationalist groups than Ukraine does.
And to pick up on the point about corruption you made earlier. Yes Ukraine has an issue, but it has been improving since Zelensky came to power. Reducing corruption was the main policy he ran the election on and he has had some success in tackling it. It's always going to be slow going when the country has had deeply ingrained corruption since the time of the Soviet Union. Even with the problems, Ukraine still has a better rank on the Corruption Index than Russia does. And I can tell you, having spent time in Russia over the past 15 years, the corruption there is rife and impacts almost every aspect of life. I've come face to face with it several times.