Russia: Invaded with less than 200k soldiers. They later on mobilized approximately 300k soldiers. This mobilization was extremely unpopular and resulted in mass resistance within Russia, Russians emigrating and so on. Since then they swapped entirely to a 'voluntary' system of deployment, with some reported incidents of coercion. Benefits for soldiers are extremely high which is likely driving a significant rate of enlistment. Soldiers are demobilized after their contract ends, which is a very important issue for a war entering into its 4th year.
Ukraine: Had approximately 300k soldiers before the invasion. After the invasion they closed their borders, prevented men of 'fighting age' from leaving, and declared a general mobilization so that anybody between the ages of 27-60 could be forcibly conscripted and deployed to fight. They have carried out this mobilization very aggressively as well as constantly lowered the minimum age, and standard, of mobilization in a country that already had a severe demographic crisis before the war. Demobilization is somewhere between inconsistent and nonexistent.
Current: Both sides seem to agree that Russia has a significant manpower advantage with an army of approximately 700k soldiers remaining in Ukraine.
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To even begin to create comparative casualty measures you need to create estimates for how many soldiers Russia was able to 'voluntarily' enlist and estimates for their contract length to account for demobilization. And then you need to contrast this against how many people Ukraine was able to conscript and mobilize. You can find numbers for these online, but nobody's even pretending to try to be remotely objective and there's highly organized propaganda abounds, so the numbers are completely meaningless.
And then on top of that you also need to somehow figure out how many are actual casualties and how many are desertions. For instance Ukraine has apparently filed more than 300,000 criminal cases over desertion. Of course they also have numerous major motivations to avoid KIA classifications. So basically yeah - you're not going to be able to realistically estimate casualties on either side, with anything even resembling a reasonable degree of confidence, until the war is over.
Ukraine is going to have some painful demographic issues to deal with when the dust settles (and I am cheering for them!).
The scariest thing is that even in the best-case scenario, this may no longer be possible. Even before the war, Ukraine's demographics were dire, then young people left, and no matter how the war ends, there's no objective reason for them to return.
On paper the US is broke. I'm not sure about the Europe situation but I doubt they're doing well either the growth statistics I saw last were a joke. They're all struggling to even arm Ukraine for this war.
If I were a Ukrainian strategist I too would not count on some sort of vast infusion of resources appearing after the war. These are not the conditions of the Marshall Plan where Europe had massive economic potential and the US was the world's leading industrial superpower with wealth to spare.